With regard to leash work, if you pull the dog, the dog reacts by pulling back. If you were to push the dog, they would push back. This is the opposition reflex. And it is a great training tool. Well we don't want that.
Owner commands the dog to sit.
By the way, the dog is wearing 2 leashes, one connected to a prong collar, one to a standard leather or buckle collar. A stranger walks up, grabs the leash to the buckle collar and pulls. Just enough that the dog shows resistance, but makes it look like he's really straining to pull the dog out of the sit position. After bad guy lets go, owner lavishes praise. In effect we have tricked the dog into staying put, but by their own choice.