How they shift

I was watching the american lemans on speed and the announcers were saying something to the effect of the racers shift without clutch for the upshift by rev matching but use the clutch for the downshift. I was wondering how they do this, i've shifted my dads truck without clutch (upshift and downshift) and on the upshift it seems faster to just use the clutch because it takes longer for the engine to get down to the proper rev to shift, i can see downshifting without clutch to be possible but someone explain how upshifting without clutch would give gains.

I saw what you are talking about, and here's a more detailed explanation:

The Porsches have electronics for a no-lift upshift. The actual shifter mechanism is cables, and though I don't know the details of the electronics, it basically closes the throttle while upshifting without having to lift off the gas pedal, allowing for faster upshifts, and more time where the engine is putting power to the ground. The gains aren't huge, but it may pick up as much as 0.75 sec/lap at a place like Road America over standard shifting.

The transmissions on those cars are totally differnet internally than on a street car, and what they are doing basically doesn't work on any street car transmission. Even though it can be done, in every street car transmission I've ever driven, it has been faster to shift with the clutch than without.