At any level of swimming, the straighter you can get, the faster you will go. Like with many tasks in life, good posture and a straight spine is important in swimming.
Stretch one arm in-front and rest the other on your side. Balance your body on your side, hips stacked, and kick.
The aim is to hold a straight spine to make your kick on side (KOS) shape as straight as possible.
Progress this drill by counting six kicks, taking one stroke to change to the other side.
Progress further with three then five strokes. Always focused on the posture and alignment.