Discovered Attack

Discovered attack a move that moves one piece out of the way to reveal a previously blocked attack by another piece.  

After knight takes the pawn, the white bishop can't take it back because Nxc4+ not only takes the pawn, but also release the rock at the same time. Thus white has to move his king first.  

Black advances the pawn and attacks the white queen, thus white has to move his queen. At the same time, black's e7 bishop is aiming on white's a3 bishop which is hanging, so black wins material because white can't save both bishop and queen at the same time.