I feel like he uses it to freak people out all the time. Like playing knife-hand, you know? Then actually slicing off a finger, acting like it's this OMG THE PAIN deal, everyone freaking out (minus those who know him well) and them him just laughing as blood spurts out like "Hahah. Got ya! Just kidding, guys. Seriously. This doesn't hurt. Come on- stop making that face. Wanna kiss it better?"