For a horse to use his back to store and return energy this way,  it must be relaxed and there must be enough energy for him to accept and return.  Too little energy and his back isn't used at all - too much and it becomes tense. Get it just right and it's brilliant!

When a horse uses his back this way its motion  is part of his stride.  It appears to be round, swinging, undulating, and elastic.  Yes, all these terms describe the same thing - the horse using his back properly.  Often, when horses use their backs correctly, riders will say their horses are "connected" or "through".  Of course it does feel as though the energy is coming 'through' the horse, and it should, because that is exactly what is happening.