What is the trick to knife-throwing?

Depends entirely on the knife. Most of the time people try to throw knives that have a heavier handle than a blade and that's just stupid... The best throwing knives are knives that have a heavier blade than a handle for example a Bowe Knife. But, if you have a knife like the one I described the trick to throwing it is to hold it by the tip of the blade and then aim at your target.