White’s queen pins Black’s queen—sort of. The Black queen is free to move along its diagonal, and to play QxQ; it just can’t move off the diagonal. Since the target is protected once and guarded once, the pin does not yet appear to be effective. But White can change that by throwing another attacker at the pinned piece with Rd1: a cross-pin. Yes, Black still can play QxQ; but this has become costly because now White has the reply RxR+—and then, after Black moves his king out of check, White still has the recapture b3xQ (using the priority of check to gain material).

The key to seeing the solution here is to notice, first, that the pinned piece is aligned both with its king and with another piece that makes a good target; and secondly that the loose piece that is the second target is aligned with its king. This means that capturing it will create a check, which in essence gives White a free move as Black has to spend his time on evasive maneuvers. The free move is critical because Black's initial QxQ is so costly; the sequence only works for White if he can avenge it and pick up the rook on d8.