Use a gun dog slip lead, pit your left hand in where the dogs head should go and tighten the loop up around your wrist. Put the loop where you should normally put your hand through around the dogs neck then pull the lead through the same part and hold it with the your left hand. When you can keep it tight to control the dog but when you let go it comes away totally from the dog but is still attached to your wrist. Easy to show do but not easy to explain.