This is a response from another forum..

"Double clutching is when you downshift by puting in the cluch, take the tranny out of gear, release the clutch and reclutch, then shift into the next gear. By popping the clutch out in nuetral you bring the intermediate shaft up to speed so it more closely matches the speed of the main shaft in the lower gear. Not necessary when upshifting really, and never necessary in any car built after the 70s (not including dog tooth race gear boxes etc)

Speed shifting is when you shift at full throttle using the clutch, power shifting doesnt use the clutch and may or may not be at full throttle. The latter is NOT to be done in an import ;o)

I almost forgot the one I use... Stutter shifting. My ECU is set up to move my rev limit to 6k rpm when the car is moving and the clutch is in, so i can shift without lifting (speed shift) and it wnt over rev. Also, since the stutter rpm is right above where where the tach falls to in a shift, it puls the car along a bit much like launching does. fun stuff..."

Another user..
"Double Clutching is still used, if you drive a semi. Double clutching occures when the manuel tranny has no synchros in it. To get it out of gear, you have to first step on the clutch. Once out of gear, you have to raise (or lower depending) the rpm to match the gear of choice. Once that is done, you have to pop the clutch again, shift into that gear, then drop the clutch. Just try any heal-toe manuver with that."