the REAL double clutch in detials:
say, 3rd to 2nd gear downshift
move to neutral
move to 2nd
thats double clutching. by bliping it in neutral, it gets the synchros going at the same speed as the gears.. not the wheels or something, so less grinding occurs.
in hondas, its pretty useless. a good example is when you go into reverse, it grinds sometimes.. rev doesn't have a "synchro" as the other gears do, and its cut straight, not on an angel. double clutch into reverse, and i bet you will never grind (at a stop of course)