My 53 has one. Pull it out and twist, and it stays. I asked my dad (since the truck is older than I am), what is this thing for. He said they used them like Red said. Out in the field - Granny gear and a little throttle and you could walk beside it and unload the hay. He told me just use it as the "poor man's cruise control" (since I'm not a farmer). Works good.
