Shock loss happens for two reasons. First, hair that is in the recipient area, that is weak and would fall out anyway due to MPB is likely to be permanently shocked out by the trauma of the surgery. Second, strong hairs in the area can be transected by the incision, which causes them to permanently die. Some strong hairs can also be shocked out by the trauma, but will generally grow back. Planting in an area with hair is always tricky. Some docs will claim that they plant in between hairs, but that's not guaranteed. There is also something called donar area shock, where the strip is taken from. You see, the hair transplant procedure is fairly traumatic to your system. Nearby hair can be shocked out and most of the time it will grow back, but I have heard of cases where it didn't.