Yes, having a horse round through its back causes the hind legs to reach further under and (usually) the head will drop. Think of the horses body as an arched bridge.....an arched bridge is stronger than a straight bridge or a swing bridge:)
Lowering their necks into a frame will not cause the horse to start using its rear or its back.
"The thought that the lowering of the neck does increases the range of motion of the horse’s thoracolumbar spine is also inaccurate....The truth is that the effects attributed to the lowering of the neck cannot be achieved by acting on the neck. Neck postures are convenient short cuts promising results that are in fact the outcome of precise coordination of the horse’s physique, starting with the decelerating and propulsive activity of the hind legs and continuing with the capacity of the back muscles..."
It's a double edged sword. Without roundness you will not have a loose, swinging back. Without a loose, swinging back you will not have roundness.
Work more on developing done pushing power in the hind legs, get a good instructor and the roundness + effective back will come. Posted via Mobile Device