Methocarbamol, sold under the brand names of Robaxin, Robaxin-750, Carbacot, Skelex (and others) is a prescription-only short-term medication used for pain relief in muscle spasms and for musculo-skeletal problems.

It was authorised for use in 1957, and there are currently 3 million prescriptions per year in the US alone.

It works as a centrally-acting muscle relaxant (rather than acting on the muscles themselves) but its precise pharmacological mechanism of action is unknown.

Methocarbamol is a carbamate with centrally acting muscle relaxant properties. Though the exact mechanism of action of methocarbamol was not established, it's postulated to be via a mechanism similar of carbamate, inhibition of acetylcholinesterase at synapses in the autonomic nervous system, neuromuscular junction, and central nervous system.

Source : US National Library of Medicine

Further info. Wikipedia