I think you are right to compare a slipper with a short thick strap. Experiments in my youth showed that the slippers that hurt most were the ones with the most pliable soles – especially the cheap ones with a black canvas upper. These acted very much like a strap, especially if the upper was cut away allowing it to flex more. The slipper seemed to store up energy as it bent, delivering it with a snap as it straightened up on impact.

Although you say that the anatomy would “greatly” affect the severity, I’m not sure the difference was so significant. If the slipper hit hard enough, I think everyone found it painful.

The other factor with the slipper is the sound. The resounding whack helps to reinforce your belief that it has hurt. It also has an intimidating effect on those watching.