Sometime ago I wrote Skier Tuck about tucking while skiing.
First, I tag this as an expert matter (despite the previous tag as an intermediate skill) because your ability to turn and stop decrease when you are tucking, plus your ability to absorb bumps and other terrain oddities decreases. So, do not tuck unless you are comfortable at the speed and can quickly get up and reestablish control.
I have found when I tuck I do it mostly correct, but I usually reach out with my arms as far as I can, it seems to me that my speed increases the further I reach out. However, watching Doug Lewis discuss tucking prior the Wengen downhill. So, naturally I went searching for a video of that demonstration and while I do not find the Universal Sport demonstration I find the next best thing a video Doug made on his own:
I will now keep this in mind and bite my gloves the next time I tuck, which I do on a regular basis at the end of NASTAR runs and at those times when I am attempting to bust speed barriers. Again, I do so only when safe.