Half the fun of dance concerts is watching dancers experiment with gravitational pull. Whereas some styles -- butoh and Afro-Cuban come to mind -- embrace the natural tug of physics, other forms, like ballet, have staged an ongoing fight against it with technique stressing time-warping trajectories and weightless landings. Like Susan Marshall's modern pas de deux in harnesses, The Kiss, and the wall-scaling work of Project Bandaloop, choreographer Elizabeth Streb has outfitted her troupe Streb like mountaineers and sent them surging and falling in staggered patterns from 300-spring trampolines, and bouncing off vertical stages like the padded "All-Wall," which the dancers hurl themselves against as if it were actually a floor. This kind of hyperkinetic movement has its own vocabulary, which Streb calls "PopACTION": a "head skudge," for example, means dragging yourself across the floor by your head. And the only sounds the audience hears during shows are the miked commands ("Fall! Jump!") the dancers utter as they careen through space, unencumbered by plot or musical accompaniment. As if the All-Wall weren't physically punishing enough, Streb raises the stakes with "Across," in which a dancer continually adjusts to the changing levels of a platform rolling across the stage on casters, and "Fly," in which a dancer harnessed into a 200-pound lever balanced by 400-pound counterweights tests centrifugal force. The show begins at 8 p.m. Friday and Saturday at Zellerbach Hall, Bancroft & Telegraph, UC Berkeley campus. Admission is $18-30; call (510) 642-9988.