There is a way to do it where you sort of let it slide out and it wants to go forward, but as you let it slide you drop your hip (keep your knees bent) and the drop can add some backspin by putting downward pressure on it, there are several ways to do this and this is just one. Another is to combine the throw with a bit of a push, you half throw half push, very hard to explain though and even harder to do at first.
You can't just throw hard like this though, you have to develop the ability to do it gently, it takes a soft touch to reduce backspin and still get the right throw. Once the throw can be done you learn to do it faster and the results become more impressive. It is said that heavier objects are more forgiving, but that the techniques are best perfected with some lighter things. I am no expert at it though, keep that in mind. I just dig throwing.
I'd expect for knives it would be good to get some grip and throw information from Ralph Thorn's book. I think I am going to end up doing that.
