"Mix mode" is a mode with random generation of puzzles. When generating a "mix", 10 puzzles are randomly selected from the entire database of puzzles while considering your account settings (Elo strength, already completed puzzles only, unsolved puzzles only, choice of section, etc.).
Since the type of exercises (checkmates, forks, enfilades, etc.) are mixed, it allows you to work your mind in a different way and to develop other reflexes
