Face down (as above), this is freestyle swimming without using your arms, which are held at your sides. And face up (separate drill), this is backstroke swimming without using your arms.
When you can perform this drill comfortably, face up or face down, with a smooth 6-beat kick, and breathing to either side (for the face-down version), you are ready to start using your arms to swim very fast, very efficient, freestyle and backstroke!
To coordinate the 6-beat kick with your body rotation, you may find it easier to begin practicing this Backstroke Rotational Kicking Drill first.