Also known to many trainers as a check-cord, the light line is an invaluable aid to training. It allows the handler to control the dog at a greater distance than is possible using the training leash.
Fifty feet of light-weight cotton awning cord can be purchased at the local builder supply for five or six dollars. Get a couple of small bolt-snaps at the same time.
Burn the line in the middle, so that you have two pieces, twenty-five feet long. Tie a bolt-snap to one end of each piece and tie a hand loop in the other end. Use a bowline knot at both ends.
It's a good idea to wear gloves when using a light line, especially with big, strong, dogs. If he suddenly decides to take off at full speed, and you grab the line, it will be stripped through your palms before you have a chance to marry a loop over your thumb. With bare hands, this can result in rope burns that will leave a lasting impression, not only in your mind, but on your hands as well. Doeskin gloves are great for this kind of activity.
I regularly use the light line to help make the transition from working on-leash to working off-leash. With the dog trailing the light line, it's easy to prevent the dog from escaping if he breaks a sit or drop. Until he has learned that I can control him from a distance, I just stay near the end of the light line. If he makes a dash for freedom, stepping on the line, or picking it up, allows me to maintain control.
The light line is also good for the dog that runs off instead of coming when called. Let him trail the line around the yard for a few minutes until he forgets about it and then call him. Just make sure you're close to, or standing on, the end of the light line so you can grab it if he tries to beat you.
There are many situations where the light line can help you teach your dog at a distance. Just don't ever leave it on him when he's not supervised.
Good luck with your training!
Neil