A laser produces a beam of highly concentrated light.  The pigment located in the hair follicles absorbs the light emitted by the laser.  The laser pulses for a fraction of a second, which is just long enough to vaporize the pigment.  This disables several follicles at a time to eliminate or significantly impede the hair’s regrowth.  However, inactive hair follicles and those containing minimal or no pigment are not adequately affected by laser treatment.

This technique is referred to as lasertrolysis.  It was invented by a team of laser dermatologists at the Wellman Laboratories of Photomedicine at the Massachusetts General Hospital.