I think it was Mormon teens that started “soaking”. It was decided by them that sex wasn’t actually sex unless you were thrusting back and fourth. Just sticking it in and not moving was fine.

Then, logically, it’s still not sex so long as you aren’t the ones trying to move while you’re soaking, so if a third friend happened to be bouncing on the bed and it was causing the motion of sex, that’s still OK too.