The reason for double clutching is that the transmission operates at two different speeds when you are inbetween gears, and they need to be at roughly the same speed in order to mesh into gear.
(Transmission connected to wheels)(gear)--(synchro)--(gear)(transmission connected to pressure plate)--(flywheel and engine)
The synchros use friction to helps the half of the transmission connected to the wheels, speed up or slow down the other half of the transmission enough to slide it into gear. Sometimes the difference is too great though and that's where double-clutching comes in. You're using the engine to speed up the other half of the transmission instead of just waiting for friction to do it's part. You still need to make sure the engine and tranny are rev matched after the trans is in gear though.
cbcbd said:
let's say you're in 3rd and want to go to 2nd:
-depress clutch
-put in neutral
-release clutch
-rev to matching rpm for 2nd at current speed
-depress clutch
-put it into 2nd
-release clutch
