A common backstroke flaw is the tendency to swim too flatly, with little to no rotation in one's body. This affects two primary components of the backstroke. First, with too minimal of hip rotation, you cannot acquire the proper angle of your catch and your pull won't be nearly as strong as if you had positioned your body properly to maximize water movement. Less rotation equals a less powerful pull.
Secondly, rotation in backstroke (and freestyle, for that matter) produces a coupling effect. Gary Hall, Sr. of The Race Club gave an excellent description of this effect in a piece on his website (seen below). I personally think of it as a bullet spinning out of its barrel when a gun is fired. The energy your body produces via rotation of the hip bone (almost out of the water) is coupled with the propulsive force of your pull, thus creating a stronger pulling force.
The next time you swim backstroke, truly take the time to exaggerate rotation back and forth and feel how much more of a handle you can get on the water as opposed to a flat 'catch'.
COUPLING – WHAT IS IT (as seen on TheRaceClub.com)
The second reason we rotate our bodies is a little harder to understand, but it is just as important as the first. I call this second phenomenon coupling. The act of rotating our bodies from one side to the other has zero direct propulsive effect on our motion down the pool. Yet when this motion, which creates energy of its own, is coupled with the propulsive force generated by our pulling arm/hand, the two forces occurring together result in a stronger pulling force than if we were simply pulling alone, without the rotation. One can consider the relationship of these two motionssynergistic.