Sorry for the late response. I had to go find some pictures we could talk from. Here is a shot of what you see from the rear of the horse. http://i512.photobucket.com/albums/t323/lostfarmer1/2009%20wagon%20train/IMG_3480_3.jpg
And the side. http://i512.photobucket.com/albums/t323/lostfarmer1/2009%20wagon%20train/IMG_3479_2.jpg
I have better pictures but on a dark horse it is hard to see the harness. With a yankee britchen the britchen goes across the croup. There are quarter straps or sometimes called quarter tugs that go from the britchen under the belly to the pole strap that then runs between the front legs to the breast strap and neck yoke. I hope that all makes sense. Here is a picture of the neck yoke-breast strap-pole strap connection. http://i512.photobucket.com/albums/t323/lostfarmer1/2009%20fair/IMG_3685_5.jpg
One advantage is the britchen is out of the poop zone. I feel the biggest advantage is in the mountains holding a load back the load in on the hip and pushing the legs down instead of behind trying to sweep the legs out from under the horse. It is said that a team can hold back or back twice the load with a yankee britchen over a box or butt britchen. I think that is one reason that this style is popular in the mountain area of Utah, Idaho, Western Wyoming, Montana, and eastern Washington. There are other areas that use them but I see more yankee than box britchens on draft horses and draft style harness. Harness is often traditional to an area. Meaning a harness maker made some and it became popular and other makers copied the harness to where it becomes the norm. That is changing a little with the advent of mass worldwide communication.
I hope this answers some questions but likely created others. Please ask away and I will try to answer. LF