(from http://www.elecdesign.com/Globals/PlanetEE/Content/755.html ) Double-clutching is a gear shifting procedure to save wear and tear on your gearbox, and on your clutch, at the expense of a little extra work for your left leg. Let's say you're accelerating in second gear. When you're ready to shift into third, you decide not to use the standard approach, which is just to tromp on the clutch, take your foot off the gas, shove the shift lever into third, and let the clutch back out. Instead, you opt to double-clutch properly: First, you take your foot off the gas and kick in the clutch. You shift into neutral, AND let the clutch out. You wait perhaps 0.3 to 1.0 seconds for the engine to slow down from its high revs, depending on how fast you were revving when you started to shift and how much inertia the engine has to slow down. THEN you kick in the clutch and shift into third, and let the clutch out quickly, feeding the gas appropriately. If you have judged it right, when you let the clutch out, there isn't any JERK. And when you shove the lever into third, the gears and engine are at a synchronized speed, so there's minimum wear on the synchronizers, which are the tiny clutches that bring the clutch plate and the gears into smooth synchrony. There's also usually less wear on the clutch plates. What's the big deal? The main point is that when you try to shift into third gear, the engine has slowed itself and the clutch plate down to the right speed—just about the same speed as the gearbox—so it saves wear on the synchronizers and the clutch. It can also save shock and strain on the whole drive-train, because the speeds are just about synchronized when you let out the clutch. Double-clutching can aslo be used when down-shifting. You have to shift into neutral and then blip the throttle momentarily before you shift into the lower gear. It requires practice and a good feel, a good touch, to do it right, especially considering the embarrassing noises you make if you miss your shift into a low gear on a crash-box. For example, you should aim to have the revs just a little high, so if you miss, the engine will soon slow down, and then gears will be at the right speed to mesh and the cogs will slip in.... Smooth transitions are everthing in racing. If you're downshifting during a turn at a road course and you're cornering at the limit, a sudden lurch in your car isn't going to be too pretty. Say hello to the curb/barrier or whatever is going to catch you when you spin out. Double clutch properly, and you'll be faster/smoother at the track. Most modern streetcars don't need double-clutching, because they have a synchromesh transmission, in which the synchronizers decide which pair of gears to connect to their shaft.