When used as a backtie, the bungee line adds pressure from behind on the dog. The dog has to bite harder on initial entry and hold that grip, or the tension from the stretched bungee behind them will cause them to lose it. It's good for building speed on entries and improving the grip for dogs that aren't biting as hard as they could be. That said, I've never seen it tied up in a tree like that before - usually just backtied on the ground.
One note - when you're buying used bungees, or even if you have a bungee you buy new that gets a lot of use - you need to check the rubber before training every time to make sure it's not cracking. Eventually, the rubber will start to (dryrot?) and crack, and if it breaks during training, there's a risk of injury to both dog and decoy. Anybody who's ever got slapped with a rubber band... well, same principle, just a lot bigger...
