I played the game again this morning, and it seems it is good for getting speed from a idle start, such as when standing on the starting line, or when you have recovered after falling off the bike. It probably has more uses, I'll let you know as I play the game more :)
You can try this: Stand on the starting line, and hold the clutch and acceleration-button. Release the clutch when the race starts, and you should get more speed than usual. You could also try it for other things, such as holding the clutch while in the air, then releasing it as soon as you land, giving you a good speed boost after a jump.
Interesting. And I confirm, it helps you accelerating when you're in low speed and you need more power. It's like in a car when you're shifting down to have more power from the engine.
And it enables you to do a wheeling !! (not sure that it's the correct word :P) At the start, being immobile (or in race at slow speed) you clutch and begin to accelerate, now keep accelerate button pushed and press down to put your weight in the back (keep the key pushed the time the front wheel rise).
It's a little bit strange that this kind of things aren't explained in the manual... But : we'll find all the clutching's secrets !