[dmurray]

A stalemate [0] occurs when the side to move has no legal moves but is not in check. Note that a stalemate is a special case of a draw: every stalemate is a draw but not every draw is by stalemate. In the final position, if Black captures the rook, it is stalemate. If he does not, White will keep giving check on the seventh rank and the game will soon be a draw by repetition[1].

This game is particularly special because before 46. Bb3 White has six pieces, all of which have legal moves, making stalemate apparently unlikely and checkmate inevitable from Black's aggressively placed Queen and Knights. It's surprising to a human that within a few moves, White is able to force Black to capture or block all 6 of those pieces and give stalemate. There are famous combinations from human play where one side manages to sacrifice "desperado" pieces for stalemate [2][3], but nothing so ingenious as this one.

Specifically, Queen to f4 forces Black to both capture the Queen and block the pawn on f3.

[0]https://en.m.wikipedia.org/wiki/Stalemate [1]https://en.m.wikipedia.org/wiki/Threefold_repetition [2]http://www.chessgames.com/perl/chessgame?gid=1252040 a famous example [3]https://chess-db.com/public/game.jsp?id=4500512.14117908.508... a less well known example I witnessed in person. An amateur woman shocks a top grandmaster.

[barrkel]

Bishop to b3 is forced - nothing else would stop Queen to d1 and checkmate. Actually, most of the game plays itself beyond 42 or so. It's remarkable that it got into that position, though.