Chin strap vs fuller-coverage mask?

I may be misunderstanding, but isn't a chin strap's purpose to keep the mouth closed and reduce mouth leaks? I'm confused by how that would work, seeing as how there's certainly no need to open the jaw to still have a significant amount of air escape though the mouth.

Personally, I build up enough pressure in the pie hole to blow open the hatches without ever parting the chompers.

Signed,
Confused In California

A chin strap CAN be successful SOMETIMES, for many it's a waste of time and money. A well fitting FF mask on the otherside is almost always 100% effective. Jim