Pyrokinesis utilizes a psychokinetic ability of altering vibrational frequencies of the atomic structure of atoms by speeding them up which causes a chain reaction of temperature change. This reaction creates heat, and if enough heat occurs, an ignition of fire can be created. Similar to the way a magnifying glass can start a fire through just the right angel with a refraction of light from the sun, an adept is able to concentrate enough to increase vibration of the atomic energy into a point of light, in which ignition can be created at will in a specific way or specific location.
Pyrokinesis can also happen haphazardly though, and without intention. To un untrained person, manipulation of a fire already lit can be achieved, such as with moving a candle flame or even snuffing it out.