Sounds like a Johnny Cash song doesn't it?
Walk the Line is a tool that agility trainers use to help their dog understand rear crosses on the flat, before asking them to do them over jumps.
Am I speaking greek to you? A rear cross on the flat is when the handler (that would be you, or in this case me), crosses behind the dog (behind = rear) on flat ground (no jumps involved).
Walking The Line is a game that helps the dog understand your hand signals and at the same time, helps you develop a consistent set of signals to use with the dog. To do Walk the Line, you first have a small treat in each hand. With the dog walking on your left side in heel position, use your left hand to pull the dog in front of you, then pivot left 180 degrees, using your right hand to signal the cross. The dog should also turn left, and should end up on your right side as you travel back the way you came. Then use your right hand to push the dog forward, pivot right 180 and use your left hand to turn the dog right.
Sounds clear as mud and I can't find a video to illustrate. But try it, it's fun!