This is an interesting training method used to get stronger finger/forearm muscles but i don't see this specific training style ever talked about for rhythm games (at least for osu). The training method is quite simple, just shove your hand into a bucket of rice and do various things like twisting your fist whilst submerged both directions, opening and closing your hands in the rice, and even diving your hands deep in the rice. This is so far my personal favorite forearm workout and i've only lightly been doing it for about 2 months but its not like i have any evidence that it makes you better but i have improved a bit (it's around the time that i have came back to the game lol) but back then it was hard for me to get progression as smooth as it is now then it was for me then. Here is an example of the workout and let me know what you think. (I am not saying by any means that this is a replacement for playing the game only just a supplement to osu mania training and also getting big forearm gains.)