I use my elbow as the main fulcrum and move the rest of my body very little for underhand throws, the shoulder barely moves. Also I try to keep the knife, or pen, or pencil, or stick, or needle or shuriken, in line with my forearm and when I throw I release and follow through instead of stopping the hand with a snap. I release the object when its tip points at my target, unless I have to adjust for range and then I may choose to change the angle of release.
If you put a rock in a sock and spin it and let go, it flies in the direction it is facing when you let go. This throw is very much like that, the release has to be very gentle so that the object moves forward in a straight line, the rock in a sock is like this too, if you let go smooth it sails forward predictably, if you let go rough it is inaccurate.
An arm can't spin like a sock, but it does have some pretty good fulcrum points that can be used to generate centrifugal force, the speed of the object is tied into the speed of the throw, however the motion must be clean and gentle to work. Using this method one never throws hard the way you would with a large mass, just fast... as fast as one can and still maintain proper form. Learn it soft and gentle and increase speed and distance...
This throw works underhand, over hand and side arm and with a few grips. I like to do it with my palm turned slightly inward but also face up, the reverse of the overhand version, again it is the elbow that moves a lot, the shoulder should not move much at all. Also the throw should be relaxed and have very little tension, if you tense before you throw it doesn't work as well and speed is cut down a lot.
It works with forks and stuff too, pretty much anything provided you keep the center of gravity of the object in line with your forearm. At least that is the technique I have been using.
