When you have to take a poo ... Really bad, but don't have access to a bathroom. The poo pokes out of your butthole as you try to sqeeze it back in so you don't shit your pants. Much like the way a turtles head peeks in and out of it's shell.
Often you can tell if one is turtle heading because they will walk funny, from the effort it takes to keep from shitting their pants
When you lightly push your anus to see if what is coming is just a fart but then you realize it's actually a poop and the tip of the turd begins to come out just like a turtle sticks its head out of its shell to check its surroundings.
Emma: Damn I thought I just had to fart but when I was turtle heading I stained my underwear.
Emily: I told you not to push that hard, now we have to buy you new undies!