Black’s queen is under attack—indeed, it is pinned. If its guard were driven away it might be taken. White looks for checks he can give and takes full advantage of the pin on Black’s queen by putting a rook en prise to it: Re7+. Must the king move? No, Black can play RxR. But the joy of doubled rooks, as White has here, is that one can replace the other: now White replies RxR+, and this time moving the king is Black's only recourse. Wherever it goes, White has QxQ.