I think what you have is a throwing stick. There is a proper name for it, but we always called it a 'throwing stick'. It's used as a lever for throwing a spear. It's use increases the distance, speed and kinetic energy of a spear. The back end of the spear sits on the 'cupped' end of the throwing stick (held horizontally) and lies along the top of it. The thrower holds the spear in place with the tip of a finger and thumb then uses the throwing stick to launch the spear.
http://imgc.allpostersimages.com/ima...hrow-spear.jpg[COLOR="black"]
---------- Post added at 02:32 PM ---------- Previous post was at 02:29 PM ----------
I believe the Aboriginal name for it is a Woomera. There are variations in shape from tribe to tribe and region to region from what I've seen.