It's simpler than the previous definitions make it out to be.
You take a finger, i don't care which one, stick it in the girl's asshole, curl it like a hook, and just start pulling.
There's no need to get bogged down in the semantics of the where and why's of the pulling. just so long as there's a hooking of the finger and vigorous pulling
She really enjoyed the forced puckering of her anus on account of the Jersey Meathook