Figure[White to move]

See how White’s f4 knight masks his queen. See that Black’s queen is loose. See how the knight can give check in one move (Ne6+). All the ingredients of a discovery thus are in place. The check looks harmless because it is so easily defused with BxN or f7xN; but by then it has served its purpose, which was to gain time to allow White to play QxQ+.