These horses have a strap under their tails called "butt breeching" by draft horsemen in Iowa. This strap is used when the horses are backing a load, or going downhill. The weight of the wagon pushes on this strap.
My own horses, here on the right, have on what we call "hip breeching." That strap is just above the tail and works just the same as the butt breeching.
Most Amish, in the Kalona area, use the hip breeching style on their work horses. The Amishmen that sold me horses and showed me how to work with a team all used this style. One of them said, "Horses like hip breeching better, they can get under a load and really push on it." I am impressed at how much these guys notice with their horses. On many occasions I've heard comments about what horses "like" and I believe they know what their talking about. The man I bought Karma from told me that she loved her collar.
Above, you can see the wagon tongue up between my horses. The piece crossing the tongue is a "double-tree" or "evener" (single tree for one horse.) You can see straps coming from the collar area, down toward the double-tree, these are called "tugs" or "traces." At the end of the tugs are heel chains, these are hooked to the double-tree. This is the mechanism for pulling a load with horses so they can share the work.
See the red pole between my horses? That is called a "neck yoke" and ties the front end of two horses together at the collar. My wagon tongue can be seen below the neck yoke, it slips into a ring on the neck yoke and is the connection between the breeching and the load, to hold it back, or to back it up.
In this picture you can see the front end of the tugs coming from the thickest part of the collar. The tug is attached to metal bars, called "hames" these hames lay in a grove in the collar. The collar is a comfortable leather oval that horses push against.
Horses actually push loads by leaning into their collars. We call it "pulling a wagon" because they are in front of the wagon, but they really are pushing.
Harness in other regions of America are constructed slight differently, and in Europe work harness looks very different. This is typical Iowa work harness.
I hope you enjoyed learning a little about harness and hitching horses. If you did, check out Amish Horses Facebook page: https://www.facebook.com/AmishHorses