Black is down the exchange and his queen is in immediate danger. He has no way to guard it. But before withdrawing it to safety he of course looks at his checks—Qe2+ or QxQ+. Either way his queen gets taken, but on tracing the lines in the resulting positions Black sees that if the two sides trade queens, the result is to leave White’s king and rook on the same light-squared diagonal with nothing between them. Black has an attacker he can use on that axis; he plays Bb7, pinning and winning White’s rook.