Name that torture device

Today was mostly a good day.  I ran a little this morning.  Didn't go over so well.
Anyway, I worked out with John the Trainer this afternoon.  That went much better. Well for a while anyway.  He decided that I needed to stretch my ass.  Stretching is not your friend.  Yes I know it's good for you but it is not your friend.
He goes and gets this tool.  It's looks like a cross between a softball and a golf ball.  It's hard with just a little give.  Take said thing, roll your body over so the side of your butt cheek is on the ball.  Then take your left foot and put it across your right knee.  If you manage to not fall off, your prize is you get to roll around on the ball until you find the spot that is going to make you cry.  You will know the spot and it hits you quickly.  I was told people either laugh or cry.  On the left side I laughed or the right side it was somewhere between screaming and crying.  It hurts so much.  A pain shot through my leg and my toes went numb.  I have done some crazy stretches before but this one was by far the worse.  I was more tired doing this stretch than I was during the whole workout.
Now it's is my new best friend.  I took it home with me.  Lucky me.
Since this thing is now a part of my existence it needs a name.  Therefore I name thee torture ball.