1… Qxg2! 2. Qxg2 Rxe2
The point is that after these moves, the white queen on g2 is pinned by the bishop on c6.
The best white can do is 3.Qxc6 bxc6 which still gives black a winning endgame position.
Black used this pin tactic to win material and simplify the situation into an easily winning endgame position, particularly thanks to the passed pawn on c6.