Generating power in backstroke comes from have a fast flutter kick and the ability to be able to use the arms and shoulders in the pull.
The greatest power in the pull comes from the body rotating on its axis, so the swimmer can take hold of the water at the top of the pull. The rotation then allows the arms to drive through the pull. Essentially, in backstroke when one shoulder is under the water engaged in the pull, the other shoulder should be up and out of the water aiding the recovery.
In our squad, we do a lot of work on single arm backstroke drill, like in this video from Speedo. The key is for the head to stay still and fixed on the ceiling while the shoulders rotate. One thing to notice in the drill is how the swimmer rotates their hand halfway through the recovery, which helps keep the arm straight and have the hand enter above the shoulder instead of out to the side.
Practice this drill regularly to improve your backstroke.