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?