21
What's black's best move?
(hexbear.net)
Hint 1
Black must avoid checkmate.
Hint 2 (a very big hint)
Black's e pawn.
e1=N+ - underpromotion!
Underpromotion is very rare. The key thing is it's a check and a check is a tempo which is a very important concept in chess. Why not a queen? No tempo :( so white has checkmate in 3 starting with Rc8+.
I watched this speed chess game live and the move happened even before I thought about underpromotion. I wonder how long it would have taken me to see it in a slow game.
