The IPL headpiece emits a heated light that is absorbed by the pigment in the hair which, when heated to the right degree, cauterizes the blood supply to that hair follicle, stopping future hair growth.
Effective hair removal requires the follicle to have an active blood supply. As you progress through the treatments, the follicle size and structure changes, requiring adjustments to be made to the machine in order to target the finer regrowth.