the potential problem with using bands is that the natural response is to want to drive the bell in a more vertical plane at the end of the movement to fight the resistance of the band, whereas without bands that's when we get the float and our emphasis on zipping into a plank can be emphasized - we see this action in the last few reps of your swing and that's when the hyper-extension is most pronounced. the natural movement of the hips in the swing should be along a horizontal plane, it's easier to maintain proper technique without the band. i would work without the band until you feel sure of your technique, and when you feel sure of your technique, try to find a heavier bell instead of adding resistance with the band, it's easier to maintain proper form without the band and practicing with proper form is what will help you advance with the exercise.