Most of the time, it doesn't hurt.
And the only way that I know how to explain it, for myself, is to be graphic in my words.
Generally, it happens when we're in doggie position and the guy likes to pull out a little bit before thrusting. Women love that and often, it's combined with a hard thrusting, that women also crave.
So ... sometimes, and this tends to happen MORE with guys who have a pyramid shaped cock, if a guy pulls out some, air enters into the vagina when he pushes his cock in.
Now, we all know that part.
But when it's a little bit of air, it's just a noise and embarrassing. Or they giggle. Or if they're with me, they start doing it more.
But if there is a LOT of air pushed into the canal of the vagina, and the guy thrusts in deep INTO that air, it hits the cervix and it's like you being punched in the balls.
A couple of years ago, a favorite client of mine was just going at it, and I was adoring him for it. And we were teasing and playing. No one's fault. But he liked the noise, and we were having fun, and he pushed into me like that and ... I had to quit the appointment.
Tears sprang to my eyes and I hit the bed on my stomach while almost curling up in a fetal ball. I thought that I could have been internally injured. Which I guess is a similar response if a guy is kicked in the balls, if he was on his hands and knees.
When that happened to me, I started asking a few of my girlfriends about this and it's a bit more common than one would think. But honestly, when someone gets hurt like that, I rarely believe that it's intentional.
In my experience, the pain part is rare. Maybe another female will have more input on this really interesting topic!!! grin
I know, graphic. But you asked.