Energy from the hindquarters, coming through the swinging back to the connection with the rein, is the Holy Grail of riding. I think though, that sometimes riders focus too much on the energy first without having a horse that will respond to go, stop and straighten from just the seat aids, then getting the alignment and relaxation, and THEN adding in more energy. If all riding should begin in relaxation and end in relaxation, then the amount of energy added makes a huge difference to the softness and thoroughness of the horse. Think about a remote control car with bent alignment. First you have to straighten the alignment and make sure the brakes and throttle work. Then you can add power. Otherwise you are spinning your car in fast circles going nowhere.
There are many different exercises in Dressage Naturally dealing with energy that I have found pretty simple and effective to either add more energy OR to deal with excess energy until you have some basic alignment and relaxation. The really the big takeaway for me though, from DN, has been that without the base of a horse that can walk, trot, canter, halt, and laterally move shoulders/haunches on a free rein pretty quietly, adding in more energy is a recipe for a braced and not through horse.