Here is some good info, with the link it was taken from, following below:
'1. Uphill or downhill?
Building on "Conformation 101" think about what you want to do with your horse. Do you need a horse that can execute fast starts (e.g. barrel, racing, eventing), or one that can carry most of his weight on his hind end (e.g. dressage)? You can choose a horse that is predisposed to certain talent by looking at the levelness of the horse's spine relative to the ground. The terms "uphill", "level" and "downhill" are used frequently, but not often fully understood. What surprises most people is that most horses are built downhill because of the avid interest in crossing thoroughbreds and quarterhorses with many other breeds. Both thoroughbreds and quarterhorses need fast takeoff speed, and to be successful at that, they are built downhill.
Question: When is a horse built "downhill"?
Usual Answer: When the croup is higher than the withers
Response: Correct. When the croup is higher than the withers a horse is ALWAYS built downhill.
Question: When is a horse built "uphill"?
Usual Answer: When the withers are higher than the croup.
Response: This is where the theory breaks down. A horse that is high in the withers is NOT always built uphill. A classic example is the racing TB. They are ALWAYS built downhill, but because the TB has a prominent wither, the wither is often higher than the croup or equal to it. Because they are built "downhill" they are able to dig in and execute fast starts. Any horse required to execute fast starts needs to be "shorter" in the front end. Picture a greyhound in your mind. If an animal is shorter in the hind end and takes off fast, they will come "up" in the front end like a speedboat, because it will be the the hind end that will "dig in". This causes the animal to lose valuable start time while he tries to lower his own front end. https://sites.google.com/site/apples...nformation-101
Thus, I can see the high whither horse not necessarily being built uphill, but I have never heard of a horse that is rump high, not being downhill