I've used bungies in 2 different ways... one wasn't really a bungie, it was a tie out with a spring in the line.... here are the ways I've used the bungie....
The purpose is to build grip firmness, teach the dog he loses the bite when he lets go... tie the dog to a tree/post in a bungie and either work myself or have the decoy make the dog miss the bite, build frustration, then when the dog bites, the decoy walks backwards n increases tension on the bungie to make the dog forced to hold on.... now, if I do this on my own I need to be very careful and not go very far so the dog doesn't fly backwards when he lets go, but if I am working with a helper it's much more effective because I can hold the dog on his harness so when he lets go, I can stop him from physically injuring himself. I think this is a pretty typical way people use the bungie.
The other thing I was talking about with the spring, let me show a photo here:
Basically the dog is on a harness with the tie-out on a swivel at the top of a tall post (like a horse walker), the decoy frustrates, then gives the dog a bite, then pulls the dog backwards... the dog is lifted off the ground with 4 paws by his grip... we did this a few times with Cujo, maybe 2 or 3 bite sessions, he went from letting go to holding on perfectly. The only downside to this is with a dog that strikes hard and high -- the dog can flip up and land on their back when the decoy makes the dog miss, so again, the handler needs to be vigilant and everyone must keep in mind the type of dog this is -- keeping the dog at the end of the line instead of letting the dog run to the end of the line will probably prevent this.
It also helps for general building of frustration.... making the dog work for the bite, and have to pull themselves against the pressure of the bungie to get to the decoy, and helps teach a dog to pull at the end of the line instead of just stand at the end of the line barking.
I'd be interested in seeing how other people use the bungie.