From Comedy Central's "Halfway Home", a ceremony in which one picks up three pebbles of differing size with one's ass cheeks.
One must then walk forward, and drop the pebbles, one by one, smallest to largest, into somebody's hand.
Coined by the big wall Yosemite climbers of the late 70s and early 80s, a pebble wrestler is a boulderer that appears to be "wrestling pebbles" from the view from Yosemite's Half Dome or El Capitan.