Turtle-Head is when you are holding in a turd for so long you can feel the poop poking out of your butt like how a turtle pokes its head of of its shell.
"oh no" johnsaid to himself during the meeting, "I can feel the turtle-head!"
So you're a man having a big dump and you're turtle heading. It's at this point that you use the poo poking out of your ass to rub your g spot in your anus and climax that way. It will require strong glutes and anus control to do this like a pro but that is a turtle wank