I just switched from gap to byrons method. I never read his book, so I could be wrong, but I think we are pretty much alike.
I think it has good hunting application, and I think it's what many instinctive shooters do without realizing it.
You concentrate HARD on the one spot you want to hit, but rather than just shoot instinctively you are really envisioning in once sense or another your arrow.
It's a bad choice of words because I don't actually envision anything. I am just consistent on form and basics and understand my arrow trajectory. Shoot enough and you know what a shot would look like without having to release the string.
It IS pretty hard to explain come to think of it, and it's probably a good book. Like pj said once you hit a certain range and your arrow drops it can lead to "huh?" moments. It'll be in perfect alignment but low. I don't practice out past that distance much either though because I'm a hunter, not a target shooter. But at any rate I feel it's a good system for hunting. It's not based on memorizing gaps which might get confusing in the field, or change due to small changes in form, etc.
You just pull back, can see where your arrow will go, and let it go. It's not based on any specific distance, target shape, or environment background. It is based on many hours of shooting the same bow though. I never noticed if the gap changed to be honest, because I don't know what the proper gap would look like to begin with. But I'll take byrons word for it.