The power anklet actual adds 4 speed evs to the speed ev(s) already gained. Not doubles. Such as if you kill a Basculin, normally it will give you 2 Speed evs. With a power anklet on it willl give the pokemon 6 speed evs. The power anklet temporarily halves the speed stat of the pokemon it is on.( if a pokemon has 218 speed, with the anklet on it will only have 109. After the item is taken off the speed will be put back to 218)