Figure[Black to move]

Notice the checks Black has with his queen: Qd1, Qe2, and Qxe5 (never overlook a check just because the needed square already is occupied by the enemy). Consider whether any of those moves also attacks anything else and see that Qxe5+ aims the queen at the loose rook on a1. The only hitch is that the e5 bishop (the e5 square, really) is protected by a pawn. Black has nothing he can use to capture the pawn, so ask another question: if the bishop is taken by another piece and White recaptures with the pawn, will e5 become available? Yes, it will. Black thus plays 1. …RxB+, 2. f4xR, Qxe5+, and then takes the rook, gaining a piece and a pawn.

In the previous two positions we captured the guardian of the needed square; here we capture the occupant of the square and invite recapture. The result in either case is the loosening of the square.