OK, I'm going to "try " to explain this. Jump in if I go wrong. Regular concentric coils are shaped like a "O" and if you could see the energy coming from the coil, in a side view, it would look like an upside down triangle or cone. Now, let's use a common 8" coil as an example. If it was resting on the ground, at ground level, the "cone of energy" (Like that?), would be 8" around, at an inch deep, it may be only 7" around. And at 2 inches deep, it may be 6" around and so on until 8" deep it's just a dot. By now you should see the need to overlap your swings. Using my example, if you overlapped your swings by half the diameter of your 8" coil, theoreticaly, you would miss targets more than 4" deep between the swings. (It's alot easier to explain this in person with pencil and paper)
Here's where the DD coil is your friend. A "DD" or "Double D" is essentially two coils side by side in the shape of two "D"'s sitting back to back. The "cone of energy", instead of an upside down cone, goes straight down untill it "bottoms out" around 8"-10". They generally go a bit deeper, and the "cone of energy" is more box shaped, thus eliminating the need to overlap your swings.
Now given this, we have all heard of the "Big Foot" coils made by Jimmy Sierra right? They are a DD type coil 18" long and 3" wide. Jimmy claims that it goes as deep as a normal 8" concentric coil, but it's an 18" long "Double D". With not having to overlap your swings, at 18" you could really cover some ground!
I hope this made sense, and if I was wrong about something, I hope you guys will jump in and correct me so I will learn more.