I use them when doing long walks, or even when dragging some chain. I want the dog to learn to work while carrying a few extra pounds. The initial mouth will be faster and the shaking will be more forceful, more violent. It is a lot like putting a weight on the end of the bat and taking cuts while on deck. The bat speed increases.
The muscles in the neck stretch and contract. maybe not so much as lifting a weight, but the weight being there and the walking, carrying, looking back and forth, nose down, nose up. On a walk the neck is in constant motion. EWO