It's a shirt, sez Slate:
The bench shirt—which comes in denim or polyester—has arms that jut out zombielike, perpendicular to the chest. The position is so awkward and the fit so tight that lifters typically need help swaddling themselves. …
Inzer says the bench shirt "brings out the deeper strength of a lifter." Powerlifting traditionalists and scientists think the opposite. Indiana University biomechanics professor Jesus Dapena says the shirt helps lifters hoist more weight by using their arm bones—not their muscles—as levers. "I would consider it cheating, it's helping you mechanically. You might as well have a pulley," he says. Even so, all the top lifters trot out the same line: "The shirt alone isn't going to lift that weight." And don't think wearing a bench shirt makes 1,000 pounds feel like nothing. Mendelson says that when he's pressing 1,000, "I can feel my bones flexing." The first time Kennelly held a half-ton he heard a humming noise and had blurred vision. "Now my central nervous system has adapted to it. I'm used to it," he says.
"I can feel my bones flexing." That's a quote for the ages.
(Hat tip to Gene Callahan.)