It's ok if your knees are bent. That's just decreasing the lever arm and making the load lighter. Do attempt to get your thighs perpendicular to the floor. It might help to press your palms firmly into the floor to stabilize your upper body. Control the descent of your legs rather than letting them kinda fall. With more control you'll have an easier time changing direction with a strong contraction of the opposite side obliques. Work on control before increasing your ROM. Just use the same ROM on each side.
There's also a whole school of thought that rotational movements performed with a stationary upper body while rotating the lower body have some potential risks. If you think you're torquing your lower back with these, then you might want to consider an alternate exercise. I am not opposed to this exercise, IF it is done correctly. The lumbar spine shouldn't be turning excessively. Get your rotation our of the thoracic spine with a tight, stable lumbar spine.
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Lisa Holladay, CSCS
Our main business is not to see what lies dimly at a distance, but to do what lies clearly at hand.
--Thomas Carlyle
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