I’ve seen them used a couple of times on outside horses up here who get summer sores in their sheath. If you lay it out on the ground, from front to back, and look at photos of it in use (like from Amazon or Big Dee’s) it’ll make sense.
There’s a small bib section that goes across the front of the chest with a neck strap, then a wither strap and a sort of crupper piece.
One hack that helps hold it in place is to add an additional strap lengthwise over your horse’s back, so that it connects the wither strap to the strap over his haunches. My friend just used a stretchy leg strap off an old blanket.