Arm-Brain is a partnership variant where one player names a piece to be moved (eg King or Knight), while their partner decides which piece (if there is more than one) and the move to play. It is a fun game, although I find it very challenging. But not as challenging as the following puzzle.
While not quite 'Arm-Brain' the conditions of the puzzle are as follows: White starts with 1.e4 and can then tell Black the type of piece they can move (again, Knight or pawn etc). Black is free to make any move with the type of piece named (so if White says pawn, any pawn move will do). Can you find a Mate in 5 for White? (NB White can name a different piece for each Black move).
This challenge was set by GM Michal Krasenkow over dinner after the Doeberl Cup had finished. Tournament winner GM Surya Ganguly solved it in around 20 minutes, while I gave up after 10.