Your almost there. The purpose of the double clutch is to spin the input shaft of the transmission.
Letís say your traveling at 50 mph in 5th gear and engine rpm is 2500. How many rpm would you need if you were traveling at 50 mph in 4th gear, letís say that engine rpm in 4th is 3500 rpm at 50 mph. Thatís 1000 rpm difference. So lets put the car into 4th from 5th at 50 mph.
1. Push the clutch.
2. Pull gear shift to neutral.
3. Push gas peddle till engine hits 3500 (we know itís 3500, because we know our car and thatís the rpm it runs at 50mph in 4th gear).
4. Pop the clutch (the clutch needs to move fast here about .10 seconds, about the time it takes to say the m in Mississippi, let the clutch out then right back in). This leaves the clutch disk and input shaft of the transmission spinning at 3500 rpm. Now we know that the tail shaft and gear assembly inside the tranny are all lined up at 3500 rpm at 50 mph. The syncro doesnít have any work to do because everything inside the transmission is all synced up. So...
5. Place the gear shift into 4th and release the clutch. Do not release the throttle from that 3500rpm point. You might throw yourself through the windshield. Unless your going for maxium effect release the throttle slowly or floor it to pass someone.
Letís say we want to shift to 3rd gear from 5th. Then we better know that we need to push the gas to 4700rpm, cause thatís the speed the engine turns at 50 mph in third.
If You donít release (Pop) the clutch in neutral and raise the rpm of the input shaft then nothing in the tranny is turning the same speed and the syncroís still have to do all the work for you. If your doing this constantly like on a track, the sycroís heat up and lose lubrication ability. This can cause the syncro to freeze to the gearset and your screwed.
There is nothing in this procedure that will help or harm the clutch. All the clutch is doing is turning a 5 lb mass for that .10 sec. Thatís not much work compared to moving a 3000 lb car.
To be able to double clutch, you need to know what MPH and what RPM, for every single gear. All cars are different. Otherwise you wonít know what rpm to tap the engine to, in order to sync the gears.
Keep in mind, this is to help your transmission stay healthy. If itís to much for you then get an automatic or just drive normal. Driving can be hard work sometimes. If you are young, it will be fun. Once you learn it you wonít even think about it anymore. As you learn the speed of your car through the gears, use your ears. We use to play a game, we would use a blindfold, let a friend drive the car and try to guess speed, rpm, and gear with our ears only. Even by yourself you car close your eyes rev the engine and guess the rpm, (car not moving of course). Your car will constantly talk to you if you just listen.
There is another technique called clutch-less shifting which makes all this very apparent, but that is a whole other post.