Unrated game where you are allowed to take back as many moves as you wish. The end result will not affect your rating or be in your stats. You can actually take back your opponent's last move if you noticed you blundered, and then your move. In fact, you can then take back even more moves, as far back as you want.
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Not sure, I only found out about it a month or so ago. To be honest, I'm not a big fan of it, but I can see the benefit for learning purposes.
What is a takeback game?