"You know I am a Parselmouth, yes?" Harry asked. "Well, there is an interesting spell in Parselmagic which prevents regrowth over magically-severed skin. You'd be amazed to know how many snakes can re-grow their bodies if not sliced at just the right point on the neck. So a clever Parselmouth developed this spell to prevent re-growth and the retribution which might follow. Here, let me show you."