Freestyle and backstroke are both “side to side” strokes in that the swimmer rotates along the spine of the body. This kick also doubles as a type of drill insofar as it develops body roll along this axis.
The kick is done without a board; fins are encouraged. Start off lying on one side with your bottom arm extended above your head and your top arm at your side. Kick about six times; then take a stroke and rotate to the other side. Repeat.
With this kick, you are effectively freezing your stroke—bottom arm in extended entry position, top arm in extended follow-through position—while you kick. As you take a stroke to rotate to the other side, focus on gradually accelerating from the beginning to end of that stroke. Finish with a nice snap of the hips as you roll the body.
When you are comfortable with the basic rotating kick; then add a sculling motion with your bottom arm (i.e. the arm extended in the entry position). Scull by medially and laterally rotating the forearm. Sculpt the water with your hand. After six kicks, move from the sculling motion into your stroke to roll onto the other side.