Shimano Rear Der Gold Lever?

Magic

Formerly 1sh0t1b33r
Team MTBNJ Halter's
#2
Gold lever turns the clutch on and off. Keep it on all the time, except when you need to take your wheel off. Press it down more for on, as it is not fully engaged.

The clutch prevents the chain from flopping around and coming off in the bumpy stuff.