to the continuous threats.

In the Birnov study illustrated, White's situation seems hopeless: they are down a piece and cannot stop Black's h-pawn, and their passed a-pawn can easily be stopped by Black's bishop. However, they can save themself by restricting the bishop's movement to set up a perpetual pursuit. They begin:
:1. a6 Bxc4A direct pawn race with 1...h3? fails, as White promotes first and covers the promotion square.
:2. e4+!This pawn sacrifice forces Black to limit their bishop's scope along the long