The act of throwing a knife at a target, be it immobile, mobile, living, dead, or inanimate. Though it can mistakenly appear to be rather easy, it is quite challenging. The problem is that most knives are not designed to be thrown, and even those that are designed for throwing purposes still suffer one rather big flaw: Only about 50% of the knife will "stick" when thrown. Unless you can thrust the knife out into the air in such a way that it torpedos to it's target without spinning or losing enough velocity to stick, the knife will spin. Because of this, unless the point or the upper half of the cutting edge hit the target, the knife will drop to the ground. Fortunately, there are ways around this.
DISCLAIMER: Before we go any further, I would like to make it very clear that you do this of your own risk. You are launching sharpened chunks of metal at high speeds. It doesn't take a brain surgeon to figure out that things CAN go wrong with this. You choose to do anything in this writeup, you take responsibility for your actions, injuries, dismemberments, loss of house pets, or whatever else may come of it. Moving on...
The first way around this is to figure out how much your knife will spin in a given distance. Let's say that you have a target you are going to be throwing a blade at. Let us also assume that your blade is of strong construction and will withstand the rigors of throwing. I'd highly reccomend a high carbon steel blade for this, as some stainless models will break after a very small amount of throwing. Now what you need to do is to stand a certain distance away, holding the knife by the handle. While it may look cool to hold a knife by the blade and fling it, you tend to miss a lot and it's a tad more dangerous, much like shooting people with your gun at a -90 degree transformation, so stick with the handle grip for now. Stand comfortably, bring the knife up over your shoulder, pointing directly behind you, and snap your arm forward. Step forward if it makes you more comfortable, but just remember to mark where you stood when you released the blade. The idea here is to get the blade to spin once through the air, and stick point first into the target. If the blade points downward in the target, step a bit closer. If it sticks upward, back up. When you get it to stick perfectly straight, measure the distance from where you were standing to the target. This is the base measurement you need to know for that knife. It should be somewhere in the neighborhood of about 20 feet, though YMMV.
Now you know that when throwing that knife, if you are 20 feet away from something, you can hit it with pretty good accuracy. But what about 10 feet? or 30? Well that is where the second grip comes in: holding the knife by the blade. It's less accurate, but for a fixed blade with no adjustment, it's pretty much the only way to hit your target at one of these odd distances. Execrcise extreme caution with this grip, as the chances of cutting yourself, even on a dull blade, are rather good if you aren't careful. Ideally you want to grip the blade with the cutting edge facing towards you, back of the blade clenched behind your first knuckle, thumb wrapped LIGHTLY around the cutting edge. Use the same stance for throwing, but make sure you either lift your thumb before you release, use a very dull blade, or wear gloves. The idea here is pretty much the same as the first throw, but you hold the blade by the handle so that you can hit that odd number of 10, 30, 50, etc. It should start at half of what distance the knife took to make one complete spin, and go up by that same distance from there. You basically reverse the knife to take care of that 50% of the blade that won't stick.
Once you have those two methods down, practice with them, and practice with estimating distances. Pretty soon you will be able to pick out a target a distance away, say, "35," and be able to hit it with the correct grip. But what if you don't deal well with set rigid instructions and distance measurement, instead going more off of intuition? Well fortunately, there is another throwing method for you, though it too relies on holding a knife by the blade to launch it. I don't personally reccomend this method, as it's not near as accurate as a handle throw can be, and it relies mostly on either your guesswork, or your near mystic intuition with your blade to be truly accurate. Either way, if you want to try it, what you do is grab the knife blade between your fingers and thumb. Draw back, and throw. The idea is that with certain distances, you either choke up, or down on the blade, to make it spin more or less. Once again, not very accurate, a high learning curve, but if you can make it work, more power to you.
Another way to get around the spin problem is to either get a set of knives with adjustable weights, or to get a double pointed knife. The weighted knives are an excellent choice, as you can calibrate them down to the foot if you'd really like, then all that's required is remembering where you put your 25ft, 45ft, and 30ft knives. You can have a wide range and never need to use a blade grip again in your life. Just make sure the weights don't break. The innovation in blade design known as the two pointed thrower, is another possibility. The advantage is that there are, in fact, two points, which ups the chance that you will get the blade to stick, but the disadvantage is that there's still a rather large window you have to hit to get the blade to stick, and you're still throwing from the blade.
Sticking with this primer and practicing the methods described should get you at least somewhat acquainted with the art of knife throwing, if not pretty damn good with some practice. The most important thing is to keep an open mind to learning this art. Learn from your mistakes, experiment, and most importantly, practice. It can be very rewarding, and can also impress your peers, and if you are ever in a pinch, it just may save your life.