Well, I'd say that "reining in" refers to pulling back on the reins to stop the horse, while being "at the reins" or "having the reins" refers to merely possess

Well, I'd say that "reining in" refers to pulling back on the reins to stop the horse, while being "at the reins" or "having the reins" refers to merely possessing them (they could be used to stop the horse, or turn the horse, or have it speed up, etc).