A double clutch (also called a double declutch) is a driving procedure used for vehicles with an unsynchronized manual transmission.
Before the introduction of synchronizers (1920s) and helical cut gears, double declutching was technique required to prevent damage to an automobile's gear system. Due to the difficulty involved in learning the technique, and because of the advent of synchonized gearing systems it has largely fallen into disuse. However, drivers of large trucks still use double clutching, as those vehicles are usually equipped with the older, more efficient, and more durable unsynchronized gearboxes.
The purpose of the double clutch is to match the speed of the rotating parts of the gearbox for the gear you wish to select to the speed of the input shaft being driven by the engine. Once the speeds are matched, the gear will engage smoothly. If the speeds are not matched, the dog teeth on the collar will "crash" or grate as they attempt to fit into the holes on the desired gear. A modern synchromesh gearbox accomplishes this synchronization automatically.
When shifting up on a double-clutched vehicle, the clutch pedal is pressed, the throttle is released, and gearbox shifted into neutral. The clutch pedal is then released. As the engine idles with no load, the rpms will decrease until they are at a level suitable for shifting into the next gear. The driver then depresses the clutch again and shifts into the next gear. The whole maneuver can, with practice, take no more than a fraction of a second, and the result is a very smooth gear change
However, in order to downshift, engine revs must be increased while the gearbox is in neutral and the clutch is engaged. This requires the driver to shift into neutral, release the clutch pedal, apply throttle to bring the revs up to a suitable speed, depress the clutch again, and finally shift into gear. This operation can be very difficult to master, as it requires the driver to gauge the speed of the vehicle accurately and is often conducted as cars in front slow down.
A related technique is called Heel-and-Toe, during which the brake and accelerator pedal are pressed by the right foot while the clutch pedal is pressed by the left foot. Note that Heel-and-Toe can be used with any downshift clutch operation, not just with double-clutching. Though difficult, mastering Heel-and-Toe in conjunction with Double Clutching is essential for high performance driving (e.g., Rallying) where straight-cut gearboxes are often used (straight-cut gears are often more durable than helical cut gears).