Port de bras on the BoSU is a variation of the original Pilates exercise of the same name on the Arc Barrel. Articulating the spine through flexion and extension helps increase flexibility of the spine while targeting the abdominal muscles more than the typical âÃÂÃÂcrunch!âÃÂàThe added instability of the BoSU triggers the deep stabilizers of the core to fire as well! The result: flat abs, a strong core and open shoulders!