A lightweight four-mover which has been used in a number of solving
events over the years.
The key, 1.Qd7, prepares to move the bishop away and threaten
mate on a4, which will compel Rb5. The specific threat is 2.Bd3 Rb5
3.Bf5, blocking the pawn and forcing the rook to move. After
1...Rxb5 2.Kb1 clears the a-file and again places Black in
zugzwang. 2...f5 is followed by 3.Qd4, threatening
4.Qa1, and the pawn prevents the rook from moving along the rank
to a square where it cannot be captured by the queen. Minor variations
include 1...Rd5 2.Qxd5 axb5 3.Qxb7 and 1...Rf5 2.Qxf5 axb5
3.Qc8.
Dafydd Johnston: I like the contrast between the two zugzwangs,
the one in the threat working by preventing f5, and the one in the main
line by forcing Black to restrict his rook by playing f5.