If White could pass his turn, Black would have to promote the pawn or play 1. … Kd1, inviting mate in every variation:

2. Kd1 Qxd2#
2. d1=Q(or R) Bh4#
2. d1=B (or N) Bd2#

To reach these lines, White needs to play a waiting move now. But this is tricky to find: If he marks time by moving the bishop, then Black can mark time by advancing the g-pawn. And if White moves the king to b3, b2, b1, or a1, then Black can promote the pawn with check. The only safe move is 1. Ka3!, and now Black must submit to one of the lines above.

From the St. John Globe, 1905.