This is drill #4 of my swim book- Simple Swims: practices for balanced, stable swimming.
This drill is key for developing the two beat kick or as I like to refer to it as pendulum legs. The legs act as counter weights to the hips to help them shift. The shifting of the hips help to drive the length of the stroke.
Start in the Front Float with a glide for momentum. Shift the hips by making the right hip lower than the left by lifting the right leg up, then switch so that the left side is lower by lifting the left leg up, the right leg goes down. Leg is up and the hip is down. Turn the hips by using the legs to shift the hips. Shift the hips back and forth. In other words, the legs are opposite of the hips. Leg up, hip down, hip down, leg up.