The science of?
Pure ballistics, of course.
As underaged said, and I think I'm the fifth person to repeat this, he's correct that the knife does not stop rotating. Any object when thrown would prefer, from the torque of release, to start rotating around its center of balance.
All objects, when spinning, turn around their center of balance. Your center of balance is located between the hips, about a few inches below the navel: check out a diver or gymnast, and you'll see this is how it's done. No matter how many flips, twists, or twirls a gymnast or diver does, the center of balance will always move in a perfect ballistic arc.
When throwing the knife, its rotating will vary by its center of gravity. A balanced blade will travel end-over-end like a wheel--right in, dropping of course with gravity (over distance). An unbalanced knife will travel in a tumbling fashion--you can see it if you watch it from the side.
There is a specific ratio for calculating the number of spins a knife will take if allowed to tumble: on average, it's along the lines of "three turns every ten feet, or one full turn every three feet." More or less, the perimeter of the blade. Keep that in mind, because it means every blade shape/length means it's a unique situation.
Now here's where Mr. Schaller's explanation plugs right in. Knowing that part about "x spins for every y feet," you need to work this out for each blade you have. As he said, you'll need to adjust your distance from the target for each type of knife--usually just a little bit. Exactly as he said, watch how it hits the target to determine whether you should move up or move back.
All you're doing in that capacity is adjusting the distance from your hand to the target so you get exactly the number of spins you need to stick it point first.
Now, as underaged added, this if for throwing by the handle. If you have a knife that its either particularly short or long, you may not want to move up or back 18"--so you can try throwing it by the blade. That cuts a half-spin out of the equation that can help you.
There are indeed zero spin throwing techniques for overhand, sidearm, and underhand--these fly like a dart into the target. In this case, your distance is not as important: you only need to adjust for ballistic drop by aiming a little higher and maybe throwing a little harder.
However, these knives usually need a lot of weight, or have some weight added to them. And the throwing technique is different--not more difficult, just not intuitive. This is never the kind depicted in the movies, of course. A rotating throw is painfully difficult to predict on a movie set, and for safety's sake, it's always easier to show the actor throw ("drop on the floor") a prop knife, and then cut to a guided knife on a wire traveling straight.
I think that's about all the physics there is. Let me know if you want more detail.
