I started to read Mastering the Core Teachings of the Buddha (the PDF, version 3).
Before page 22, there's some text which says that (I paraphrase) sensations are fleeting and that consciousness-of-sensation is consciousness just of the echo of an already-previous sensation.
Then, on page 22, there's is a description of an exercise, which I quote below.
He introduces it as being "useful for jump-starting and developing insight into impermanence"; later, says that this same exercise promotes concentration and stability; and finally says that it was the only technique he used for five years.
Can you tell me more about this?
Is it some hybrid/mixture of insight and concentration?
Is it at unusual exercise? It seems to take 'two-pointed concentration' as its object, which I haven't ever read about before.
Are you aware of any specific advantages or disadvantages of this technique (or, whether and when it ought to be recommended or not recommended)?
Is there something it does well, too well, not well enough, not at all?
In one of these exercises, I sit quietly in a quiet place, close my eyes, put one hand on each knee, and concentrate just on my two index fingers. Basic dharma theory tells me that it is definitely not possible to perceive both fingers simultaneously, so with this knowledge I try to see in each instant which one of the two finger’s physical sensations are being perceived. Once the mind has speeded up a bit and yet become more stable, I try to perceive the arising and passing of each of these sensations. I may do this for half an hour or an hour, just staying with the sensations in my two fingers and perceiving when each sensation is and isn’t there. This might sound like a lot of work, and it definitely can be until the mind settles into it. It really requires the concentration of a fast sport like table tennis. This is such an engaging exercise and requires such precision that it is easy not to be lost in thought if I am really applying myself. I have found this to be a very useful practice for developing concentration and debunking the illusion of continuity. You can pick any two aspects of your experience for this exercise, be they physical or mental. I generally use my fingers only because through experimentation I have found that it is easy for me to perceive the sensations that make them up.
In another related exercise, I do the same sort of thing, sitting quietly in a quiet place with my eyes closed, but instead I concentrate on the sensations of the front and back of my head. With the knowledge that the illusion of a separate perceiver is partially supported by one impermanent sensation incorrectly seeming to perceive another impermanent sensation which it follows, such as the sensations in the back of the head incorrectly seeming to perceive the sensations of the front of the head which they follow, I try to be really clear about these sensations and when they are and aren’t there. I try to be clear if the sensations in the head are from the front or the back of the head in each instant, and then try to experience clearly the beginning and ending of each individual sensation.
This practice also requires a table tennis-like precision. Half an hour to an hour of this can be quite a workout until the mind speeds up and becomes more stable, but this sort of effort pays off. When I am engaged with this practice, there is little room to be lost in thought. I have also found this a very useful practice for developing concentration and debunking the illusion of continuity and the illusion of a separate self (more on that later).
The section ends with (one page 25),
For five years of my practice I was basically a One Technique Freak, and that technique was noticing how sensations flicker. I would do it as often as I could, i.e. basically whenever I didn’t have to be doing something that required concentration on the specifics of my life. I would be riding an elevator, just trying to see when I could feel each foot, or lying down to sleep and noticing how many times I could experience the sensations of my breath in each second. I also tried to notice this aspect of things for every single sensation that occurred during my formal practice. I used lots of objects, usually those that were presenting strongly at that time, and would use some variations on the above techniques as well as some others that I will mention shortly to keep me from getting stuck, but the aspect of my world that I tried to notice, things flickering, was always the same. I found that by making this sort of commitment to understanding one of the most basic assumptions of insight practices I was able to make fast progress and gain the ultimate insights I was looking for.
Earlier (e.g. on page 20 and 21) it was talking about becoming aware of some number of sensations per second ("maybe up to forty times per second") which he calls "vibrations", and combines with a breath meditation,
[...] It is also useful to check out exactly what happens at the bottom, middle, and top of the breath if you are using the breath as an object, and to examine if the frequency stays stable or changes in each phase of the breath. Never assume that what you have understood is the final answer! Be alert! Explore carefully and precisely with openness and acceptance! This is the door to understanding.
One last thing about vibrations: looking into vibrations can be a lot like any other sport. It can be thought of the way we might think of surfing or playing tennis, and this sort of game-like attitude can actually help a lot. We're “out to bust some vibrations!” as a friend of mine enthusiastically put it. [...]
I searched this site for the word "vibrations" thinking someone might have written something from this perspective already (if this were a well-known technique or "basic dharma theory"), but no, I don't see this mentioned in any previous topic.