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Discussion Starter #2 (Edited)
What is "rev-matching"?

The general idea of "rev-matching" is to make for a smoother and quicker downshift. When normally downshifting, there is a pause and an unpleasant jerk as you come off the clutch pedal. This is because the rotating wheels and gearset are coming into contact with the engine's flywheel which is spinning too slowly for the new gear you have selected. The result is the engine being forced up to the proper speed. This is bad in a racing situation, where it can destabilize the car while trying to turn.

"Rev-matching" involves using the throttle to bring the engine up to the right speed before releasing the clutch, removing this undesired behavior.

  1. Depress clutch pedal
  2. Bring car out of gear
  3. Touch throttle to bring engine to the speed you expect when you engage the new (lower) gear
  4. Engage gear
  5. Release clutch pedal
 

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Discussion Starter #3 (Edited)
What is "double-clutching"?

"Double-clutch" or "double-declutch" downshifting is "rev-matching" with an extra goal: bringing the layshaft to speed by releasing the clutch pedal for an instant BEFORE bringing the car into the new gear. This will allow for smoother gear engagement when shifting and was/is necessary on vehicles without synchronizers (race cars and big-rig trucks, for example).

  1. Depress clutch pedal
  2. Bring car out of gear
  3. Release clutch pedal
  4. Touch throttle to bring engine to the speed you expect when you engage the new (lower) gear
  5. Depress clutch pedal
  6. Engage gear
  7. Release clutch pedal
Note this is just an additional two steps. This means you can rev-match without double-clutching.
 

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Discussion Starter #4 (Edited)
What is "heel-toe"?

"Heel-toe" downshifting is the application of this technique while braking. It is a common technique on the race track, where you want to be in the right gear to exit the turn before you enter it. That way you can keep your hands on the wheel and your mind on the turn as you execute it. It is explained in detail here.

A video of Gary Sheehan heel-toe + double-clutch downshifting at Infineon (formerly known as Sears Point) in his WRX can be seen here.
 

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Discussion Starter #5 (Edited)
Do I really need to learn this crap?

No. NONE OF THESE TECHNIQUES ARE NECESSARY.

On the street, the only point of learning to rev-match is if you are aggravated with the car bucking forward as described at the top of this thread. You will not hurt your car by failing to learn these techniques. If you are not on a race track with a stopwatch, there is little point in learning heel-toe, although many people feel a need to impress their friends.

The only point of double-clutching is to match the destination gear's speed with that of the driveshaft so it engages smoothly, and this is what a synchromesh already does for you. Proper double-clutching will in theory prolong the life of your synchros because you are reducing the work they have to do. However, don't be afraid that your transmission will die without double-clutch downshifts. Modern passenger cars are developed with the assumption that the driver would not be employing these techniques (and few drivers do).
 
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