Hairs have three main growth phases: Anagen (growth), Catagen (transition), and telogen (shedding)

Here you can see the anagen phase on the top row and the telogen phase on the bottom row.  Anagen hair is the best phase to catch when doing electrolysis, because it is actively growing, attached to the root and the blood capillaries that feed it. We treat it all in one shot, and these hairs may not return. If they do, then they will be thinner and lighter the next time they come in, and will be easier to treat.  The telogen phase hair already has another hair growing underneath, so when we treat this hair, in about a week, the next will pop up.  This is why electrolysis is not "one and done". We have to catch each hair in the growing phase possibly several times before the hair is completely eradicated.