Each hit with the stick has a similar feel to getting punched to the body, drains you, except it's much lighter. However, the compounded effect eventually feels like some big bodyblows until you can't take it anymore. You can do crunches in a way that doesn't so much tire you out but gives your abs that sensation of pain instead so it needs to rest to relieve that pain. This is similar, except he's not resting until it gets too painful. It must work for him because he did a ton of these against Cotto, one of the most vicious bodypunchers in boxing during that period.