How do muscle relaxants work?
Check out the muscle relaxant article on Wikipedia, it's pretty straight forward.  In short, there are two main types: Neuromuscular blockers, than act at the junction between the neuron and the muscle; and spasmolytics/antispasmodics, which (mainly) act on the central nervous system to reduce excitation or increase inhibition.  Most of the ones I've heard of, such as diazepam, are in the latter category.