Creating 'rules' or boundary cues when playing tug with your dog is a great way to keep your dog from getting overstimulated and keeping those shark teeth off your arms and hands. An important point to note about the game 'tug' is that it is a predation substitute for the grab-kill component of the pre-sequence in dogs. For this reason it is sometimes hard for the dog to let go of the toy immediately when you ask them to. They are simply sating an instinctive need, so try not to fight them for the toy or think them 'stubborn' for not listening to you immediately. Give them a moment or two to work through that instinct and then try again. You'll notice how, in this video, once Willow has thrashed around with the toy a bit, she no longer wants to play tug and is ready to move on to something new.