I'm certain it's been discussed before but my search or my key words or something wasn't too successful. I'd appreciate it if anyone can point me in the right direction, or, alternatively, start a discussion here.
Here's the little bit that I know from a few mentions I've run across in other sources. It emphasizes a line. It is used to encourage the dog to drive deep and (consequently) it isn't used on a check down bird and it's a rhythm/timing thing on the release for the last bird down.
Is that it, or is there more to it?