It's a drill where four guns are out in the field, and the marks are thrown where the line to one mark will be through, adjacent or next to the area of another mark.
For instance, two of the guns are long. One long right, one long left.
Two of the marks are short, and in between the other marks as you see it fron the line.
The long left gun thrown a flat mark from right to left. (90 degree throw). You run your dog to the mark. All ABCD marks are singles.
You then have the short left mark throw a long single flat to his left, your right. You send the dog.
The next mark is long right. he shoots and throws to his right, your left. This mark will be exactly over the fall of the mark you just threw. Send for a single and now the dog must go over an old fall.
Basically, you can set this drill up where the line to marks are right off a short guns back, under the arc, through a fall, etc. It teaches a dog to ignore other white coats that are out in the field and to just concentrate on the mark being thrown. It also helps on head swinging.
I do this drill with birds and bumpers. When I use bumpers I like to throw them farther than a regular bird throw, as it promotes a dog marking the mark and not the thrower.
This can be conusing without a diagram, and I'm sure somebody has one they can post.
Tim