A party game that two men take turns playing in a crowded atmosphere. The men pretend to be talking when they spot a girl with a nice, tight butt and the man with his back to her backs towards her until they make contact. He then moves his butt across hers from side to side and maybe even a little up and down motion. While the two men pretend to conversate, the "Rub-ber" will say, "Aw yeah?". He repeats this process, getting louder in volume and frequency to display to his buddy how nice her ass feels. Childish, but a good way to pass time.
Jen must have been oblivious to Mark and I playing "aw yeah" with her. Eiher that or she enjoyed it so much she didn't say anything.