I would guess its the same as

electrolysis.
Hairs grow for a little bit, then once they reach the right lenght they go dormant. If you pluck

them, or they fall out, they start growing again. Hair is only vunerable to electrolysis when its growing. Zapping

it in its dormant phase will make it fall out, but it'll grow back again. Zapping it when its growing will kill

the follicle, and it will never grow back. So when getting electrolysis you have to shave or pluck the area, then

zap the hairs as they start to grow back. They don't all grow back at the same time, which is why it takes a few

treatments.