Backstroke leg kick most important component is to lock your knees and kick from the hips.
You probably will end up bending your knees slightly but aim not to. If you’re not used to kicking this way you will feel this working your hip
flexors and the tops of your quads.
Try to keep the hips high. As with front crawl its easier to achieve this by pushing the back of your neck/ top
of your spine into the water rather than physically forcing your hips upwards.