After cycling for a few weeks, things have stabilized about 10 days ago. Practice is easy and deep with a good capacity to pay attention to subtle vibrations. There is pretty much always a vibration on my forehead. Pressure is building up in my head, especially in the area of the frontal lobe. I also don't need to practice much to develop a pinching sensation in the middle of my brain. It was suggested by BrunoLoff to not practice too much when this happen and allow a few weeks to relax the area:
Tips to get stream entry

Is there techniques I can use to keep pushing despite those phenomena? The sensation in the frontal lobe seems to not be a problem but the way I understand it I need to avoid the pinching sensation. Is that correct?

Drop the idea that something needs to be avoided or 'done' to that area. Whatever normal technique you are doing continue to do it but in a more relaxed non-expectant nor 'knowing' way. That is to say, don't place some expectant idea on what is being observed. Following the same technique but perhaps including the notion behind this pointer could help stop fabricating the blocks to continued practice which arise due to expecting and placing pre-conceived ideas/concepts/beliefs on phenomena being a certain way, or wanting it to be a certain way, trying to control how phenomena is experienced:

http://thehamiltonproject.blogspot.com.au/2011/10/yogi-experiment-riding-wave.html

Put a small gentle smile on your face and give that head tension permission to just be. Relax into it. Let it spread. You may be allowing your mind to contract around it, to resist and oppose it. This may in turn be causing the over concentration resulting in the pinching sensation.

Do this in addition to Nick's suggestion.

I don't know if you have stream entry yet. If you can detect the pulse coming from the middle of the brain, then perhaps the following information is of use: recently I was contacted by a guy who ignored my recommendation of being not overly focusing on that area, and in fact did quite the opposite, spending up to ten hours a day in meditation, perhaps half of these just observing this pulse; after three-four weeks of this, it seems, he got stream entry.

For him, stream entry happened when that part of his mind slowed down, and it suddenly seemed that the pulse was happening in an infinite open space (his words), and then there was a time-gap, followed by the usual post-fruition and then post-stream entry stuff.

So perhaps there is really nothing to avoid. At the time I wrote the recommendation to avoid this sensation, I was having quite severe pain and tension in that place, which would worsen to debilitating levels if I practiced even for half an hour.

I personally find Nick's advice ("riding the wave") hard to follow, as the segregating process of the mind stil happens all over the place. Also there was some point when trying to follow this advice was completely impossible, rather than just hard, and in fact would result in further conceptualization, which, as you probably know, is not the point at all. It should perhaps be added that by the time Nick could simply "ride the wave," he had practice meditation quite intensely for over nine years (if I get the number right), and was accomplished in attaining all sorts of refined states of mind.

Yes, I probably overlook and do not consider the past conditioning in place to be able to do the riding the wave thing successfully. HAIETMOBA is useful for recognizing a few fleeting moments of that if one is able to simply allow the mind to recognise exactly what is arising as one asks it. Learning to let go as opposed to grasping at the resolution of some 'thing' is also another way. I guess if one finds they cannot do any of the above, they will have to learn the hard way, meaning the intense amount of effort that can result in 'tension' in various places in the body including the head must go on until there is a natural 'giving up' or 'surrendering' of such 'mental exertion' out of sheer frustration and/or exhaustion (which I have had personal experience with and has actually led to baseline shifts).

Thanks for those comments. I will keep doing everything to stay in the present moment at the sensate level without creating unnecessary tensions. Up to now, I would say that my practice was pretty much "riding the attention wave" without using one specific object. Now that my attention is very strong, it feel that to get deeper I need to do some concentration practice like paying attention to fine vibrations. The movement of attention alone is too coarse to be my sole object. Those tension are very tempting objects of attention. I will keep experimenting but always in the direction of relaxation of those tensions while keeping attention as broad as possible. Hope it make sense.

Edit: I'm pre-path by the way.

I'm surprised that the theory that stream entry (and every fruition?) happens when a spot in the brain relax for the first isn't more common. Judging from my progress, it really seems like things are going in that direction. Is there more information than can be found on that in modern or tradionnal texts? It seems that Buddhist texts didn't talk much about the brain and this kind of technicalities. Maybe more in another traditions?

The release of tension seems to have to happens in that order:
1. limbs
2. Along the spine
3. The neck
4. The brain

It seems to me that if I don't attend at those tension in that order I end up creating more tension. A trick I'm using with interesting results is to stretch vertically using the muscle in my neck. Ultimately, I get to stretch the tension inside my brain in an expensive way. I get more and more equanimous by do so in the right order.

Simon: How long do you tend to each part?

I cannot really say that I focus on a part a specific amount of time. I try to have my attention as broad as possible and let it move around freely (well, not completely free, a little effort is need. I come back on this). If there is a tension, I will put a slight emphasis on it to be sure I don't block it from my awareness. I do this until the tension is gone. It can take a few seconds to a few minutes.

An exception is the trick I do where I stretch the base of my skull. I did this for long period of time, maybe up to 20 minutes and even more. It's the same effect as letting your head down in front of the elbows so the weight stretch the neck and spine.

I get to a point where I can practice a kind of third gear practice where I just abandon my awareness to whatever arise. I can only do this if I released all the body tension and that my concentration is very strong. What happen is that the tension in the brain will spasm too violently if there is still tension. Without the tension, I can let the area at the base of my skull pulse and stay attention to whatever happen in my body.

Seeing with the ears meditation has helped me with the tension between the head and neck. Stretching helps with the tension from the outside but this exercise helps from the inside and to prevent it from occurring.

http://www.dharmaoverground.org/web/guest/discussion/-/message_boards/message/3220835

How does that exercise go, exactly? By your short description I really couldn't figure out what exercise you are doing.

Now that the tension is no longer there, I cannot do it anymore. It involved the muscle at the base of my skull.

Things keep evolving and now it feel more like a pressure in the whole of my head instead of specific tension. I can still spot some tension point that I can feel vibrating but it's more subtle. I tried to let the expension/contraction that happen at each heart beat happen freely. Beside that it's basically noting and noting and noting. I feel a somewhat cognitively impaired, memory and speech being affected.

I also used the expansion/contraction that happen at each heart beat as a point to focus on which helped. Before using it as an object of awareness, there was expansion/contraction of the visual field as well with each heart beat which stopped afterwards due to awareness practice.

I think that mindful stretching can help with severe tensions that are held in the body.

I guess Simon has not replied to you about how the exercise is done. If this is so, I will try to describe the exercise which can alleviate head tension. If Simon has different take on it, he may describe his method to you.

Just stretch the spine with your head moved to the front so that the chin is closer to the chest and do it mindfully or with noting. I think you couldn't figure out the exercise because Simon wrote "letting your head down in front of the elbows" which is a bit confusing. He may be doing this exercise with his hands behind his head with arms parallel to the torso to push it in which case the head will be in front of the elbows. But then he goes on to say that the "weight stretch the neck and spine" which seem to suggest that he is not using his hands to stretch the spine, so not really sure.

In any case stretching the cervical spine mindfully or with noting will help with head tension. Anything that feels like tension in the brain is most probably muscular tension rather than something to do with the brain itself as Simon has found it as well. It will go away with stretching mindfully.

You may also try lion pose yoga for the head tension with the addition of the mindful or noting component.

When trying this I ended up doing 1 of 2 things. Sometimes I would end up visualizing what I hear. Sometimes I would end up visualizing eyeballs in place of my ears. They are both interesting visualization end they would shake things up a bit but create more tension then releasing them because I'm not very good at visualization and it requires too much effort. I guess we cannot do wrong something that is impossible but I'm not sure this is what the exercise is supposed to be.

Yes, this is what I meant. I also pointed out that you can do the same stretching without bending your head forward just by stretching the muscle.

Guided meditation on death can also help with any sort of physical tension which has an emotional aspect to it. Here is a link to one:

http://diydharma.org/guided-meditation-death-awareness-brian-smith

"Feeding your demon" technique can also be useful in this kind of tension.

Voku Hila describes it:

"In this technique you visualize your demon and its "personality". Then you jump into the demon and find out what he needs. After that you feed him with what he needs(love, compassion, peace)."

http://www.dharmaoverground.org/web/guest/discussion/-/message_boards/message/3269828#_19_message_3265278

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Lama Tsultrim describing the technique in video:

http://www.youtube.com/watch?v=q66Z7dhT97c