The hand represents the state of Michigan and they're pointing to where Detroit is within the state. Supposedly a hand looks like the state of Michigan with the fingers and whatnot, and being the Murder City or Motor City Machine Guns, they're pointing out where they're from.
Having it explained makes it seem like a dumb thing, but nothing Shelley and Sabin do together is dumb and if it gets people talking about them, I'm all for it.