With my engineering background I always relate things to engineering mechancis.... And your body and the guitar are basically to mechanisms that work together to create something.... Basically if you just use your wrist you're simplifying the mechanism... Less moving parts, less energy equals improved efficiency ( in engineering that basically means the amount of energy used versus the amount of "work" put out). So you're basically using less energy... shortening the stroke of your arm and making the "system" more efficient............ Basically what everyone else said... just in a slightly different way.