Typically when a horse is hollowing his/her back they don't feel so great while being ridden but this must be a trick question. When a horse is hollowing their back, they typically are not through with their gaits; there is a disconnection from back to front. Usually gaits are irregular and difficult to sit. In dressage, we must teach the horse relaxation, to carry himself and us more efficiently so that he/she can is rounded from back to front, and with evenness of rhythm and regularity.