Gear Second is where Luffy uses his legs to pump up his heartrate to levels that are normally impossible. His veins are able to withstand this, thanks to the Gomu Gomu no Mi. During the Enies Lobby Arc, Rob Lucci mentions that this is just like steroids because his muscles are receiving enough blood to do things otherwise impossible. The after effects of this is that Luffy's body turns to tween sized proportions. Rob Lucci also noted that Gear Second does damage to Luffy's own body and in turn reduces his lifespan.