How the M&W Hand Clutch works on my H

I have two tractors that I enjoy very much. One is a 1949 Farmall Cub, the other is a 1952 Farmall Super M.
The Cub has a Woods belly mower on it and I use that tractor to mow my lawn, pull trees and wood around, etc…
The Super M is a restoration project and is [...]

on my antique Farmall M tractor, ther is a M&W hand clutch. i dont really know what it is, so i wanted to know what it is.