A sexual act. A Chicago cross-jostle is performed thus:
The woman, bent over a chair, bed, or with her hands against the wall, is penetrated from behind by the male. The male places one hand on each side of the woman's buttocks. The hands are then alternately and rapidly moved up and down, causing the buttocks to jostle. When the male achieves a harmonic equilibrium of the buttocks, the vibrations are transferred to the vaginal area, causing large amounts of stimulation to both partners.
A true Chicago cross-jostle is not achieved until the skilled male rotates the axis of the jostle to 45 degrees from one side to the other. This maneuver causes the buttocks not only to jostle up and down, but also to bounce off each other.
Ample gluteal fat deposits and skill are required to perform this maneuver.
Well, he had her up against the wall, and he started in on a Chicago Cross-jostle, One-Handed!!!