The only thing keeping your colon from coming out of your azzhole is your sphincter muscle. When a girl gets fycked in the ass, obviously the sphincter has to relax to allow the penis in.
What you saw was the whole "gaping" thing some pornos do. Where the guy will fyck a girls ass for a while, then pull out quickly and the girl pushes down like she's trying to take a shyt. Because her sphincter is stretched wide and relaxed(from the penis), when she pushes, her insides(colon) actually comes out a little bit.
Hope that helped.