There are two very different views on "double-clutching" I've heard. One is just power shifting, being called double-clutching. The other is what racecar drivers do, because the transmissions used in Nascar cars and such aren't synchronized (don't have synchronizers to speed up or slow down the gears when shifting).
You really shouldn't have to double-clutch in a standard street car, as most manual transmissions these days are synchronized...some I've heard are only synchronized on one set of gears, some on all... I'm not sure.
But really, double-clutching is just, when you shift from one gear to another, pressing the clutch in to shift to neutral, and rev the engine to maintain the correct speed, then shift to the next gear and let the clutch out. Someone please correct me if I'm wrong, though. And like I said, you shouldn't have to do that on street cars, though there are applications where it is better to do that while racing. (ie: most imports, I've heard)
