The Human Torch's nervous system generates a field similar to his sister's (the Invisible Woman) that allows her to manipulate the electrostatic fields around her (imitating the effects of solid matter). However, his field is adjusted to cause combustion. The basic point is that the Human Torch's nervous system has been infected by a basically electrical field that both can start and prevent the combustion process. Since it is hooked to his nerves this allows the Human Torch to control it as if it were part of his body.