I've seen this to. There is a how to in the old American Boys handy book, by Daniel Beard. ( The original dangerous book for boys. It's a wonder any of them survived)
  Apparently it is quite effective at short range. He recommended it for small game.

ahh so it's sort of like the spears I used to throw as a kid using a piece of cord. Only you use a stick to hold the string and a much smaller spear.
different, interesting :-)

Sort of like a flexible atlatl.

yep that's what i used to throw as a kid :-)

But my dad used to make them from a small sapling instead of a bamboo cane.

You need to keep tension on the line as soon as you lock it round the shaft. It's really only the tension that hols the cord on that's why once the arrow/spear reached the end of the cord it is released as the tension is also released. maybe that's what you were doing wrong ?

The American Boy's Handy Book is actually pretty handy. It has lots of fun projects, especially novel weaponry.

Just don't kill yourself with it. 

could be - I'll have to try it with the cord on a stick. Only ever used them with the cord held in hand. You hold the spear right at the top and lock the string just above the flight. And the throw is actually more with the string than throwing the spear itself. But they did used to go a fair distance :-) I vaguely recall I bodged one up last year for the dog to chase. Used a stick I found in the woods and my sling (well it's a long cord with a knot at one end :-) Worked - but would have been better with flights on the stick lol