I'm no expert, but this is how I understand it. Here's an example. You're in 4th gear and want to go to 3rd gear. Press clutch in, shift gear to neutral & release clutch. Jab the throttle with the right foot, Press clutch in and shift to 3rd and release clutch. why not just press the clutch in and jab the throttle?? I think they call that rev matching. Double clutching if done right reduces wear and tear on your clutch and synchros. A mate of mine is a master of double clutching and has gotten 170,000 K's out of the original clutch. He doesn't drive like a granny either.