When landing on the back on the trampoline, the spine is usually slightly curved upwards. The head and the shoulders generally do not impact the canvas.
The back may hurt if one lands on the canvas with the spine straight and the shoulders impact the canvas at the same time as the backbone. The weight of the shoulders including gravity acceleration may bring about spinal disc compression if the backbone and the back are rigid and straight at the moment of impact.
In trampoline exercise, landing on the back is often to end a rebound and to end a session of straight upward rebounds.
When landing on the back on the trampoline, the head should be held high to avoid impact, the torso and backbone should be slightly curved upward so that either the butt or the mid back makes contact with the canvas at the moment of impact.