DOUBLE CLUTCHING:

There are *three* separate spinning entities that need to be coordinated when shifting: The engine. The transmission input. (I'm going to refer to this as the intermediate shaft). The transmission output (which is directly related to the vehicle speed).

When the clutch is disengaged (pedal pushed down) and the transmission is in neutral (such as when shifting between two gears), the intermediate shaft is essentially free spinning. In normal shifting, we rely on the synchro's to control the speed of the intermediate shaft as it engages with the gears connecting it to the transmission output.

Decades ago, transmissions didn't have synchro-mesh. (Many large trucks still don't). On these transmissions, it is necessary for the driver to manually control the speed of the intermediate shaft so that it matches the speed of the gear to be engaged. This is done by the following process when shifting from one gear to the next:

1) Power is removed and clutch is disengaged (pedal down).
2) Transmission is shifted from original gear to neutral.
3) Clutch is re-engaged (pedal up). (Driver now has control of intermediate shaft speed by controlling engine speed).
4) Driver 'blips' throttle to match intermediate shaft speed to speed of new gear. (This takes practice to get the right match).
5) Clutch is disengaged (pedal down).
6) Transmission is shifted from neutral into new gear.
7) Clutch is re-engaged (pedal up) and power is applied.

Steps 1 - 3 can be done casually or quickly. Steps 4 - 6 *must* be done quickly so that the intermediate shaft doesn't slow down again before it's engaged. If step seven is also done quickly, the engine will also be 'rev-matched' to the rest of the driveline so that engagement will be smoother.