The move is great, and seems to be wining on the spot.
Black queen can no longer stay on a1-h8 diagonal, which she can't leave either due to mate threat.
If eg. ....Qh8, then Rg3+, Qxg7#.
If ....Qd6 then Rg3+, Kh8, Qg7#
If ...Qg7 then Rg3 winning the queen for the rook.
How good, or bad, is this move, and why?