Have you ever rode a bull? Perhaps a mechanical one when you were drunk and there happened to be a fair on in town that day? Trying to control the flow of magic is a bit like that, wild, impossibly strong and ready to throw you to the dirt if you slackened your grip for even an instant. The magic hit Handy with the force of a wave concentrated into a single point, he had to shut off its access to the rest of his body,  to any other means of exit, giving it no choice but to flow straight into his heart.