I have ridden these for 9 years, successfully. The reason they work (at least for my family) is that they conform to the shape of the back as it moves and changes shape. They are not entirely TREELESS.
There are two tree pieces, the pommel and cantle, and are connected between the top and bottom of the saddle by a V shaped rigging. This helps distribute weight evenly. So, if you are standing in the stirrups, the weight is spread out…
Keep in mind, a traditional tree can only distribute weight evenly if it fits the back exactly, and how can this happen when the back changes shape as the horse raises and lowers his head and bends his body?
Anyway, I have 5 sport saddles, and use them on all my 5 horses…. I find them lightweight, extremely well balanced and my horses travel better. I ride with a loose group of 12 or so friends who all have them, on a variety of horse colors and shapes.
I have NEVER seen any back sores, pressure marks or white hairs caused by these saddles. I cannot even begin to list the number of people I know that used traditional treed saddles for YEARS, only to find damage; that switched me to the sport saddles.
It’s essential, I believe, to use a Skito pad with Equalizer foam. This pad is shaped to the back and has a small air channel down the spine.
I check my horses backs after any rides; have not found soreness, and at the few endurance rides they do, they are A’s on their backs.
Best example of successful use is Karen Chaton, who has THOUSANDS of AERC miles in Sport Saddles, including Tevis and the summer XP rides, day after day.
I find that the normal stirrup position is too far forward; my saddles have them moved back 2″ under me.