Teacher positioned himself, and then called out the additional instruction “touch your toes” — and he then proceeded to wham home the slipper just as the boy’s hands reached their lowest point and his trousers were tightest. And when the boy stood up after the single stroke, his face was almost the colour of a beetroot.
Incidentally, it’s interesting to think back about class attitudes to such punishments. Usually, for a one or two stroke slippering, it would be “serves you right”: but for a (rare) punishment which really hurt and left the boy openly crying, sympathy would definitely be on his side.