"It reacted to the threat and stopped it," said Harry. "But you'd been damaged, and I thought that's what the hole was until I detected Dolohov's signature in it. Then I realised that it wasn't your physical heart that Riddle thought he had damaged ... but your metaphysical one. Riddle could never have your unswerving devotion while this enchantment was in place - which he demands in all his followers - and my magic would have stopped you being able to feel at all, rather than allow Riddle to poison you against me.