Thank You Jamie Jackson for this beautiful example of a feral hoof and the scoop or arch in the quarter.
The arch or scoop in the quarter is naturally formed in the wild by the way the walls are worn down to sole level. With the heels a bit higher than sole level, this is natural wear but is also achieved while trimming by us doing the same as what would have happened in the wild.
In the cadaver picture below you can see the hoof walls are level with the sole level.
Always remember, we are only doing what nature would have done, if the horse was living as a feral. Therefore do not think about rasping a scoop, you’ll just see it there when you’re at sole level. It is my (unproven) believe that it has to afford the hoof wall more flexibility, so I like to see it there and it occurs naturally with this trim.
Besides that, it keeps the hoof walls off the ground and therefore keeps the connection between hoof wall and pedal pone tight in the quarters. When a hoof is not trimmed in this way, a clear bulging at the coronet occurs due to the quarters pushing up.
Here is what it looks like after trimming. Also note that the Mustang roll comes all the way around to the quarters since the horse uses the quarters when he turns while moving faster.