Pump your pecker in and out of a girls ass cheeks.
1. get a stiffy.
2. compress her ass cheeks.
3. insert your stiffy between cheeks.
4. pump stiffy in and out of cheeks.
Note: lubricating the cheeks facilitates pump action. "its like titty fcuking but with her ass cheeks". alternative to penetration.
"If she doesn't have big tits then try slip dogging"