In witches and wizards, your magic is just as important as blood. It transported magical energy through as everyone knows that magic and electricity do not mix. When part your magical energy gets your body. Magical energy is basically what neurons use instead of electricity in muggles, it basically causes the cells in your body to "malfunction" and die. As your cells are dying, they cause immense pain and everywhere gets damaged just as fast. It is the opposite to the Cruciatus curse, which forces foreign magic into your core. The nerves are overloaded and the excess magic causes waves of pain. Even though they cause the same amount of pain, the type of pain is different. The Cruciatus causes waves of sharp pains which travels from the head downwards: branching out. Closing part of your core stops magical flow everywhere simultaneously and it feels like a drowning pain which intensifies the longer you are denied. During this, you are twitching uncontrollably and you feel the irrational need to stop casting magic; even though you aren’t. You do not need to close a lot of it to cause pain, and the amount of magic needed is extremely taxing; luckily, though perhaps unluckily for Pettigrew, Voldemort had it in spades. However, it is impossible to kill someone with this method. It would require someone with the magical prowess and stamina of someone like Merlin to kill someone with this method.