I bought this to extend the 4 foot Zero Shock leash. While it serves the purpose of making the leash longer, it also adds weight to the leash with the additional metal ring and clasp that makes it feel awkward to use. I would prefer to just be able to have a 6 foot leash.
Also, one thing to consider with the bungee feature of both the leash and the extension is that it does give your dog some extra room to pull away from you if you stop short. So while the feature helps in some cases, it can give you dog room to move a bit forward if you stop short of an obstacle and can't grab the traffic control part or below the bungee part fast enough. If you add this extension to the Zero Shock leash, you are doubling the areas of expansion on the leash. Unless you can quickly grab below the lowest bungee part, you can't easily stop your dog from moving closer to an object, person or animal you are trying to keep them back from.