He performs perfectly the one way only (that by words), but the other is injured. As a man walking on one foot, or a carriage going on one wheel, is injured, his sacrifice is injured, and with the injured sacrifice the sacrificer is injured; yes, having sacrificed, he becomes worse.

In plain words: If the silent priest breaks his silence, only the spoken part of the ritual is done right and the silent part is harmed. It is like a man hopping on one foot or a cart on one wheel. The ritual is damaged, and the person paying for it ends up worse off.