I've found a distinct difference between my note-taking habits at judo and my note-taking habits when dabbling in Brazilian jiu-jitsu or boxing. I think my method makes sense, though I could certainly be rationalizing.
(Remember--I'm not accomplished in any of these disciplines, so taking the opposite of my approach might be called for!)
I rarely take notes for my twice-weekly judo classes. I used to, but it seemed to be a distraction from just buckling down and doing the work. (That was true even if I took notes after class instead of during.) My note-taking tended to devolve into either "this is a way to do X" or "this is the way to do X", both of which were less helpful than just visualizing the technique and how it felt.
I think a large component of this is that judo's actual syllabus is rather simple. I understand that the gokyo and list of official and non-offical but recognized techniques, including groundwork, makes the number close to two hundred. But over and over, we see that there are a few techniques that are wildly more successful than all others: osotogari, seoinage, kouchigari, uchimata, haraigoshi, pick-ups, jujigatame, kesagatame, sangakujime. Some others are common but clearly not in the same class, such as sasaetsurikomiashi, or ouchigari. The art is sublime and complex, but limiting one's practice almost entirely to this subset is a very successful and fairly common approach.
The real place that things get hairy is variations and gripping. There are thousands of these techniques, and in my case they don't seem to get names. "Billy's uchimata", "that grip break from camp", or "Bobby's kouchigari" is as close as we get. Hayward Nishioka emphasizes the importance of such unnamed techniques:
The problem with not having a name to tag a technique with is that it becomes less recognizable and hard to study. With out a name it’s sort of like meeting someone interesting but you didn’t get a name or a number. This happens more so with other means of overcoming an opponent other than throws, pins, chokes and arm bars. These areas include counter moves, combinations, advanced tactics, which include an area that is ever more important today than in the past, gripping skills.
You will most likely report to others, “[Teddy Riner] did a powerful osotogari,” when in fact he did much more. If you are a coach what will you be preparing your athlete for? Will it be the osotogari, or the gripping techniques leading to the one handed technique?
Since my instructor focuses on classic Japanese judo, these unnamed techniques remain unnamed. We practice set-ups, gripping, combinations and so on, but they are more transient than permanent techniques. We try them, play with them, then discard them if they don't fit into our game. I think this is because judo is so fluid and fuzzy. Things happen quickly in randori, and are hard to pin down with words or diagrams. The movements that distinguish Bobby's kouchigari from the kouchigari in the gokyo involve the whole body, but are subtle. Describing them in notes is hard.
If I come across a particularly stunning revelation, I might jot it down if it's confusing, or I've been doing it wrong for years. I'll put it in my training blog if I can make an interesting anecdote out of it. Usually just remembering that a possibility exists is plenty.
Brazilian jiu-jitsu notes
In contrast, BJJ is an art with a huge list of even basic techniques. Sweeps, passes, submissions, transitions, and hierarchies abound. Just getting your head around all the very different possibilities for each position takes a good chunk of brain space.
More importantly, BJJ techniques are easy to break down. The art is sublime and complex, like judo, but it is much more sequential. "Take this grip. Pivot your hips this way. Swing your leg this way. Roll on top. Make sure your leg goes over here during the transition. Slide the knee through. Establish the side control."
For me, BJJ lends itself to note-taking, so after class (never during), I jot down the steps of the technique and any sticking points that I ran into. Reviewing these notes has been fairly productive for remembering techniques.
I also find that the process of writing the notes is more productive than the process (often omitted) of reviewing the notes.
Using the axes of simple/complex or sequential/diffuse discussed above, one would expect my notes on boxing to be sparse. However, the other variable involved is simple training frequency.
I don't train in boxing at all. I occasionally do a session with a friend, and I'll hit the heavy bag once in a while. Training this is not.
Because I box so infrequently, I feel the need to cement the information more formally. So after my friend shows me a couple defensive head movements, I'll write them down. I don't train or test my boxing frequently enough to know if this is helpful, however. (NB: my BJJ is fairly infrequent as well, which is another factor in notes being useful in my practice of it.)