I wonder if I am deploying the characteristic of no-self correctly.

What I practice is feeling the self, and I often find it as a contracted, subtle, tense feeling behind and in between the eyebrows.

Then I perceive that sensation with as many "frames per second" as I can (impermanence), and then weird stuff happens.

I often feel as if I have released an energy block; tingles and rushes, everything appears blank. Sometimes I feel as if my sense of self is more spread out in my body and surrounding atmosphere.

I can't tell if this is correct, fruitful practice, or if I am simply supposed to observe that (false) sense of self.

Intellectually I can grok a small contraction is not my self, yet as soon as I am unconscious of all this, it returns like a stubborn muscle knot.

There are different ways to approach this, and it seems to me like you might be playing with more than one approach. That can complicate things, but it can also be very fruitful. I would personally encourage you to continue to explore the way you do and find out for yourself, empirically, what is what. The way I see it, that's a wonderful way to gain insight. 

Just remember the thre characteristics when you investigate, so that you don't buy into a story about a specific knot actually being the self. Also notice, that when you use attention to investigate the knot that feels like a self, there is another knot that thinks that it is the self that is doing the investigation. Notice how that too shifts when you try to look at that! 

So yes, investigating these knots is a valid practice. Emptying them out is necessary too, but that can happen in different ways. Roughly speaking, it can happen sort of by brute force when you study them so intensely that contraction increases until it has to collapse — or it can happen by simply (but not easily) letting go and sort of zooming out. Both those approaches are valid. If you find that things sort of get into a rhythm where contraction and expansion alternate, it might be the case that it comes naturally for you to also alternate the approaches. That's fine! If you are more the type that does one approach at a time, systematically, then it's probably less confusing to choose one of the approaches for the time being. 

I don't know if any of this makes sense. It's past two o'clock in the morning here and I really should be sleeping. 

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I find this approach helpful

https://www.dharmaoverground.org/discussion/-/message_boards/message/24889549


(More at the link.)

The three characteristics are interrelated, so that is a clue that the sense of self is impermanent (in this case constantly changing, arising, passing) and intricately woven into the characteristic of dukkha - notice how the sense of self is at the root of so many unpleasant emotions (aversions and cravings - jealousy, envy, hatred, ill will, conceit, defensiveness, etc etc). 

You can observe the sense of self or dukkha and it's pretty much the same thing. I find it easier to notice dukkha because emotions are things you feel in your body (physical sensations that accompany emotions) so you can learn to sense them, be mindful of them, very easily whereas the sense of self can be very subtle and easy to miss. But once I notice the dukkha I am reminded to notice the sense of self too. The emotions act like a signal to remind me to notice my sense of self. When you do this you are also studying dependent origination.

I recommend looking for the inverse of the three characteristics, sensations that seem like self, permanent and satisfying instead of fabricating non-stuff. The realization that sensations are empty of a permanent fixed self that satisfies will happen at awakenings when they are gone, until then work with what you have.
Good luck
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@dreamwalker That's a neat way to flip it, I will try that out next time.

@jimsmith Now that you mention it, I have been kinda avoiding emotions, for some reason. Sometimes it's like investigating makes them disappear, sometimes more intense. It's strange stuff to tinker with.

@pollyester Understand completely. It's nice to hear both approaches can work, sometimes this practice is so weird I lose my common sense.
I went ahead today and busted that sense of self contraction-know as many times I could, and it seems to work well to progress insight. It's as if there is more clear space in my head, more bandwidth to bust vibrations, and then observing impermanence gets automatically easier.

Sounds good.

Would you happen to be flat-footed? Might miss the mark entirely, but that could provide an alternative explanation of the nervepains

Learning to notice the physical sensations in the body that accompany emotions helped me to become more aware of emotions. I remember being at work once, noticing I was in a bad mood, and trying to think of why, and figuring out it was something that happened three hours ago. So I had a long way to go. But when I learned to notice the physical sensation it changed things completely. With practice you can learn to notice the instant emotions arise, noticing the change from the tranquil state produced by relaxing meditation, and a lot of the time you can just decide to stay relaxed without suppressing anything. What used to seem to be involuntary now seems to be just a habit  you can give up, you realize dukkha is something you are doing to yourself.