Do you mean the chin strap, which is present on every motorcycle helmet, or the chin bar across the face of a full-face helmet? I presume the latter...
sorry about the typos.
the chin strap.
if you hit a full face on the lower chin the head is jerked back causig the snap. It is a specific liability problem of full face helmets. (picture if you will a nice upward directed of a baseball bat impacting the lower chin point of the helmet. The whole helmet moves back suddenly with the chin strap acting as a noose.)
It was first discovered during the 1950's when the navy was trying to figure out why their pilots who ejected into the ocean were turning up dead from broken necks. It resulted in design changes.
The same effort is more difficult on full face helmets which have added weight.