It's all on automatic and diff ppl respond differently, but most men the balls pull up during orgasm at least, if not sooner. Room temp could affect this somewhat, making scrotal skin stay looser. Mine are quite pronounced, tightening up with deep crevices in the scrotal skin. I've seen this on porn pics of guys shooting off. Maybe the porn you were watching the guy wasn't really having orgasm.
This.. many times I think its to tight, scrotal sac shrinks though the room temp is warm. my erections pull scrotal skin sometime, especially during really hot sex activity, to the point Testes pull up into the body where its somewhat painful, and I tell my wife to pull my nuts down when doing oral..
The balls constrict while highly aroused and during orgasm. You can trick your body by pushing your balls closer to get off quiker or pulling them away to prolong. It feels awsome to pull on them while having an orgasm. I like to use a heavy ball weight that is just a metal ring around my balls to pull down on them.
She=Is it all the way in? He=It's all the way in,except my balls. She=Hell,throw them in too--they're good. I worked with a guy that said=" I'm goin' home and slap my 'ol lady in the ass with a sack 'a nuts!" Crude,but succint.
The more aroused I get, the tighter my balls get. If you want me to show you just how tight they get drop my a message. Generally during the first moments of stimulation they start to recede and my scrotum starts to tighten up. Then the closer to orgasm that I get, the tighter they get.