In two words: destructive interference.

If you and a friend hold a rope taut, and your friend raises and lowers their hand once very quickly, a wave will travel on 'top' of the rope to you. If you move your hand down and up quickly, a wave will travel along the 'bottom' of the rope toward your friend. If you and your friend did this at the same time, when the two waves meet in the middle, the rope will be completely flat for an instant.