ok.. even if you can shift without using the clutch pedal, why would you? the only way that i could see a reason for doing it is if you are uncordinated and cant shift fast.. i shift hella-fast with using the clutch pedal..
have you ever been cruising on the highway in 5th gear and some stupid honda flies by you, and you want to show him how much of a ricer he really is? what is the fastest way to accelerate? to shove it in a lower gear of course.. but the only way to do this without putting stress on your clutch is to double shift..
instinctively most people shove it into a lower gear without prepping the engine, therefore making the car jerk before it accelerates... but ... to get rid of the jerking and add a boost on the acceleration, all you have to do is shift out of the gear that you are in (ex. 5th) into neutral, let go of the clutch pedal, rev the engine, and then quickly press the clutch pedal and put it in to the desired lower gear...(ex. 3rd skip 4th) let go of the clutch pedal and accelerate like hell.. compared to normal downshifting, it feels like you just added craploads of horsepower onto your engine..
i may be wrong.. but everything that i have ever read or heard, says that double shifting is an extreamely good technique that can be used in every day driving and will extend the life of your clutch
"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."
taken from
http://www.happytogether.com/318ti/notebook/shifting/