In this puzzle http://en.lichess.org/training/58454, the computer said that the fourth move pawn a2 to a1 is a wrong move, but can someone tell me whats wrong with this move
I think it's because of the underpromotion f8=N+. He promotes with check so he gets a free knight for a moment. Of course that is still completely losing and shouldnt fail the puzzle, but I guess that's the reason.
