I was wondering the same thing. I found this, may help, may not. Hair growth occurs in 3 distinct phases: growth, regression and resting. During the growth cycle the hair literally grows and gets longer and the root is surrounded by many blood vessels. This is the best time to try to "kill" the hair. The second growth cycle is called the regression phase where the hair is present but has stopped growing, this occurs with repeated treatments. Since this hair is already “dying†and is not being nourished by blood vessels surrounding the root, it cannot be “killed.â€Â This hair will eventually fall out on its own but stays imbedded for a while to give the body a little extra protection. The final growth cycle is call the resting phase where there is no hair present at all and put simpy this represents an unborn hair, which means that follicle has not been "killed". This hair cannot be killed until it begins to grow.