Strange that it should take a Canadian to make San Francisco audiences squirm again, but The Underbelly Diaries may be the most frank and disgusting show on a local stage this season. Aaron Berg is an ex-bodybuilder who shifted one day from weightlifting to paid masturbation, then moved on to stripping and other adventures in the Ontario skin trade. He tells about his first job jerking off in front of an Austrian homosexual, for $150, with a black bodybuilder named "Hot Chocolate" (who advises him, "For that kinda shit, next time -- $200"). He talks about his work as a rent boy for horrid, husband-hating rich women. And he gives a hilarious rant about the side effects of anabolic steroids (baldness, back hair, acne, sterility, impotence, flashes of anger and sadness). Berg works up to fugues of intense comedy, and he delivers a cut, well-crafted monologue. His male voices are also seamless. The show is a major confessional masquerading as stand-up comedy, but it doesn't yet work as a fully developed solo show because Berg is so overcontrolled. He tends to muscle through lame punch lines, ignore the moods of his audience, and go in for aggressively shocking material at the expense of more human stuff, like his romantic life (if any) or his Jewishness. One joke suggests acres of unexplored territory. Berg imagines his grandfather learning about his exploits: "Did ya hear about the nice Jewish boy who looks like a neo-Nazi and threw come at an Austrian queer with a schvartze, no less?" Grandpa wants to know. In the end, really, we don't.