Sorry, I have been too busy to get time to answer this as promised. It is a very interesting topic, and deserves a thorough explanation. As usual I would like to illustrate this with film clips, but I am very bad at producing instructive movies, there is always some details that are not perfect enough, and those are always distracting from the point I want to make.

Rising trot is basically invented to allow riders that are not so good at sitting trot to let their horse's back move correctly. There is basically nothing wrong with sitting trot, the only fault is that very few riders have the skill, the balance, the stabilization of upper body and the suppleness around the hip joints to let the horse's movement through.
The younger the horse and the weaker the back, the more sensitive the horse is for a blocked rider's hip. A very strong, seasoned horse, or a large, insensitive horse, or a horse that is big and strong in comparison to the rider's weight, might be able to withstand the blocked hips, and move correctly anyway. For all others, a rider that not yet is able to let the horse through the hips will have to ride rising trot regularly, to make sure that the horse again can get its back in order.
The horse's back needs to be
stabilized for it to be able to move elegantly and with correct rythm in all gaits. This means that all the muscles around the torso/belly has to be active, and all the time has to work to hold the midsection of the horse stable. It is wrong to think that the back should swing like a jump rope, it should move more like a rather stiff, but elastic thick pole. What is crucial to understand, is however that this rather stiff but stable midsection is moving in a wave-like movement, almost like a log of timber in rough sea. You get the picture? It is possible to move in a wave movement even though the log is stiff?
Okay, now we place a rider on top of this rather stiff and stabilized midsection, in a saddle. If the rider is stiff in his hip joint, his pelvis and buttocks will block the saddle in a horisontal position at all times. Even though he might sit rather still because he has taught himself to wobble in his own midsection. The pelvis and buttocks are still forcing the saddle to keep a horizontal position.
Imagine now the log moving in the waves. Or the horse's back moving in trot, which is a very similar motion. If the saddle is blocked in a horizontal position, the horse's back is blocked in a horizontal position in the exact area under the saddle. In order for the horse to move in walk, trot or canter, he will have to make some changes to his back posture. He might do one or more of the following:
1. Sway and wobble his back under the saddle or behind the saddle, so that the spine is "broken" in two places, allowing the back to wobble, just like the rider's back is wobbling. The gait rythm will be lost over time because this weak back cannot carry the cadence if the movement. Horse over the bit or curled behind, depending on the rider's "skill" with his hands.
2. Freak out and run, trying to get rid of the problem. Buck, kick, whatever.
3. Start developing a habit of being sluggish and move without energy.
4. Move with hind legs more collected than the rest of the body, and lose its gait rythm in a different way, often called a "positive" rythm fault, but the back problem is still there.
5. There is also a lot of other symptoms that can be seen, such as forging, stumbling, bad temperament, fear, skittishness, concentration problems, etc.
From this, it is obvious that we need to keep the horse's back stable. And the horse very much likes to do so, as long as we allow him to. Thus, it will take time (duration depending on the strength of the horse) for the back to "break" again, after we have together made it stable.
This is the reason why it works rather OK to ride rising trot regularly. Even though your seat may be blocking the horse's back, you can undo this by rising every time you feel the horse "breaking" i. e. the rythm is lost.
A better solution would of course be to develop a seat that really allows the back to move without blocking it. A prerequisite for this is to be able to move through the hips, which I have described more in our riding theory. If you can do this, you can actually heal a "broken" back by just forcing it to move correctly, just by balancing the pelvis dead vertically and letting the seat bones move in the correct, circular movement that the back needs. You might even force the saddle to move like that, and thus help the "broken" horse find his rythm very quickly.
I would say that there is no way to really collect a horse in rising trot. Collection is not about pulling the reins and squeezing with your legs. It's about collecting your body and letting the energy from the hind legs lift the horse up instead of pushing it forwards. This has to be directed by the seat, and if you lift the seat out if the contact more than 50% of the time, the collecting effect is accordingly weaker, even if it still is there. For many riders, the rising trot will prevent a much larger fault of blocking the back, of course, but this does not mean that there is a rule that sitting trot is bad for the back or that it is only for very strong horses.
You can actually make the horse stronger much faster by riding sitting trot correctly.
Hanne