I believe the first clutch is for the gear engagement, the second clutch either with break or gas pedal is to match your speed to your rpm during turns, it helps you to slow down and accelerate in control during the turns. If you down shift and don't match your speed to your rpm with the correct gear then the car will perform engine break with an uncontrollable front dive which will alter your control during turns, similar goes if you want to accelerate through a corner. It has been used in WRC a lot.
You have to double clutch a manual transmission car, unless the transmission has synchros. Clutch in, put the car in neutral, clutch out, rev the engine to where the engine speed will be after the gear change to get the transmission moving at that speed, clutch in, shift into the new gear, clutch out.
Isn't that a bit like saying you need to illuminate your house with candles, unless your house is equipped with electricity? Haven't synchros been standard for a very long time on consumer vehicles?
Except in European taxis and one car a friend of a friend had in high school, I've never in my life been in a vehicle with a manual transmission that had synchros. They're essentially non-existent in big rigs, and nobody I know except big rig drivers has a manual.
You don't have to double clutch or put the gear in neutral to rev it higher, you can do all that with the first clutch engagement, especially in the straights! I recommend down shifting on certain speeds you choose and see where the rpm falls for each gear and adjust your throttle and rpm during the first clutch engagement! For upshifting you can power shift no problem! Double clutch on staright ways putting in neutral especially will cost you 2 seconds and the soccer mom driving a suv behind you will end up infront of you in no time!
I do this 99% of the time when slowing down to make turns and I always thought I just was driving my manual wrong because everyone always says "you always want to be in-gear," but it felt natural so I kept doing it.
You are talking about rpm matching the engine to the speed of the rear tires/drivetrain.You don't have to double clutch or put the gear in neutral to rev it higher, you can do all that with the first clutch engagement, especially in the straights! I recommend down shifting on certain speeds you choose and see where the rpm falls for each gear and adjust your throttle and rpm during the first clutch engagement! For upshifting you can power shift no problem! Double clutch on staright ways putting in neutral especially will cost you 2 seconds and the soccer mom driving a suv behind you will end up infront of you in no time!
There is a noticable difference in how easy the shifter goes into gear with a well executed double clutch on the downshift (matches the intermediate shaft speed). Totally unecessary with a modern manual transmission with syncros especially if you are already rev matching on the downshift, but you can still feel it ease into gear easier.
