Taken from a game I played, clearly my opponent did not expect this to be a critical moment.
It was not obvious what I played, but I did some calculation, after my calculation I found that white should be close to loosing (okay that might be an exaggaration, but clearly worse).
My move 1...Qa5+, one point you have to calculate is the terrible line on 2.Nc3 b4 3.Ne4 b3+ it does not matter where the knight goes on the 3rd move, it drops a piece.
This can then be discarded, then the next point would be Nd2 here I can just take a pawn on g2 and finally Qd2 runs into Bb4!, this is however a little more complicated so I will give this as a line.
1...Qa5+ 2.Qd2 Bb4 3.Nc3 Rc8, here black should be doing quite well.
One point I do not understand is why my opponent resigned after 4.0-0 Bxc3