Training in 10-15 minutes I have a question that can probably also help others: how to train having 10 or 15 minutes? And I am not talking about ONLY 15 minutes per day. No. Imagine that you've done your tactics today, played one game (maybe even with long time control), analyzed games or read a book. And you thought you are done.

And suddenly in the evening/night you come across 10-15 minutes of free time. E.g. waiting for a film to start/delivery guy/a spouse and would like to use it for chess progress. I reject playing blitz during this time as I do not consider blitz games as training. What do you propose? Solve a study? Play endgames against a computer? Quick opening refresher? Maybe short movies on YouTube?
It really depends on what area of the game you're focusing on improving. 

If you want to improve general tactical vision, do a bunch of "cheap tactics". These are tactics that are easy and should take under a minute to solve, and doing many of them helps to improve your implicit tactical abilities during a game (you're able to see basic tactics quicker).

If you're working on your precision, studies are a good choice. Another option is to play through one grandmaster game on the board, and try to understand the main ideas in the game. For example, looking at a Karpov game can improve your pattern recognition for techniques on squeezing the opponent.

The point of studying chess is to make certain skills implicit knowledge... you should be able to subconsciously know certain things during a chess game. Doing cheap tactics improves your implicit tactical skills, and doing a study/looking at master games improves your implicit pattern recognition abilities.