Jonny Dub, Kev and I used to play "Butts Up" in the racket ball courts of our Middle School. I was looking up the rules on Wikipedia . We didn't play all of these alternate rules. The only one we used was:
In one variation of the game, a runner who does not reach the wall before the thrown ball hits must, in addition to receiving an out, stand facing the wall and allow the thrower to "peg" him or her with the ball (usually with all possible force).
In my opinion that's the only way to play the game. I don't remember using outs either, but we could have. The main focus was on tagging a person in the back or butt with a tennis ball as hard as you could. All other rules were secondary.
I thought for sure there would be some great Butts Up games on Youtube. Sadly there aren't any great games captured on video. This one is long, but the kid is funny how he can't stop himself from looking. He also breaks the number one rule, you can't look or dodge the ball..