Flat Sidewalks

Your arm is a sidewalk.  A nice, smooth, flowing slab of pavement.  This means your wrist and your forearm create a flat, unbroken surface.

Is your wrist pointing up?  Well, then there’s a bump in your sidewalk and any poor traveler who hops down that path is likely to stumble.  Is your wrist pointing down?  In that case that same poor soul is liable to fall down that sharp hill you’ve made.

Punch strong, punch safe, punch metaphors.