What you do not want is the peak of the rump set behind the hip bone prominence. You want that peak to be over the hip and a little forward of the hip doesn't hurt. In the weak back examples, the "coupling" (Lumbo area) is concave and the LS joint is "late" (behind the hip bone). A true "peak" like the first two horses exhibit is rarely put together with a strong back... a strong back shows a smooth transition between the croup and the back.. there is no defined point for the peak of croup.
In the Roach, there is a clearly convex area ahead of the hip.. almost like the peak of croup is far ahead of the hip bones. A slight roach is strong.. and a bit stiff.. but those horses, while not as good at shifting their weight to the rear and lengthening the back to get the hind legs under the horse (see Dr. Deb's article and refer to the horses that are collected). A horse with a slight roach is not a horse that is typically retired for back issues.. and the do not get sway backed.
In the correct coupling you would be hard pressed to find the peak of croup because everything is smooth. The LS joint is over the hip bone and the tail is not set too low or too high.
In the photo of the three horses with good couplings, note that the grey is probably the best conformed horse of the lot. The middle horse, the bay, is a yearling.. but he is very strong and will improve with age.. likely developing a bit more wither. The grey has a better overall hind leg.. lower hocks and better angles (a good hind leg and a good LS joint are helpful for any higher level work you might want to do with a horse). The blood bay Arabian (or perhaps he is a Morgan) all the way to the right may be the weakest of the three (and he is very good) with his croup being a bit flat and his back a bit long which is compensated for by very low hocks and what I suspect is a stellar hind leg with good bone. Arabians often have a very strong coupling.. but paired with a flat croup and a high tail set, they actually do not always have a powerful hind end and may even have a weak (concave in profile.. behind the withers) back
In the case of Lonrho, he has an extremely short back. Some argue he is a bit roached.. but I see tremendous strength. His son may have a slightly more correct LS but he does not have as correct hind leg.. look at Lonrho's bone and the placement of his hock.. nice and low.. and his stifles are low too but he has a long gaskin. Another thing to note on Lonrho is his shoulder.. it is nearly perfect with the point of shoulder to elbow angle being neither steep nor flat. His son is the same way with perhaps a slightly better neck. Both he and his son have a ton of bone and great front legs.
As an aside these are mostly photos that are very good to judge conformation from as well. I wish our conformation posters would take photos of their horses set up like this.. camera image plane parallel to the horse.. horse standing with space to the knees and camera opposite the horse's hip.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer