The mistake trainer turns your own game mistakes into training positions: replay the exact moments where you blundered and try to find the correct move, with Stockfish guidance when you need it.
Training on positions from your own games is one of the fastest ways to improve, because you are fixing the errors you actually make. Analyze a game first and your mistakes are queued automatically.
Generic puzzle sets train tactics you might see someday; the mistake trainer trains the positions you already got wrong. Standing in the same position a second time — this time knowing there is a better move — is how a one-off blunder becomes a pattern you recognize before you play it.
The queue grows as you analyze more games, so the trainer naturally focuses on your current weaknesses. Clear the queue regularly and you are doing targeted repetition on your own errors — the closest thing to a personal coach that a free tool can offer.