Superman w/ "soft" flutter...
I use this drill for warm-up & cooling down. Also the drill is good for studying your hand strokes - keyhole stroke for butterfly & the round elbow stroke for butterfly/freestyle - its mechanics, muscle interaction, position, etc., while underwater.
This drill is vital in studying the concept of balance - the interaction between the head, the feet, the arms, the propulsion...
Also, it will initiate you into certain breathing format.
The feet/head relationship. As your feet sinks, with the soft flutter (no splashes, please), you sink your head and up goes the feet. Sometimes you help the head to sink by hand pushing it down, gently please, at the back. But intentionally hanging it lower, gently is enough. This trick - hanging the head - is what you will do eventually to correct yourself as you do the freestyle (yup, there will be moments as you do your freestyle, slow mo, that the feet will be lower than your body. You either increase the intensity of your kick, flutter - whichever - to correct this. Or just simply - as taught by TI - sink your head, and automatically the feet rises up.
The hands. I move the left in a gentle, no splashes counter clock-wise 6 inch diameter rotation movement (also play around with its position/diameter - just under the water in front, slightly to the side, increase diameter of rotation to wider/smaller diameter. All the while you are gently fluttering.
To breath. Increase the hand rotation diameter (as diameter increases, upward pressure also increases), change the position of your rotating hands, and bring your head up, gently, just enough to clear the mouth to breath. Do this several times if you need more air. You will be nodding in the water. This face front breathing will initiate you to the breathing done in butterly, low-amplitude breast (what TI calls the "sneaky breath").
To assist in lifting your head to breath, another option is to gently with your hands press, put pressure downwards (remember your hands are in front of your head, in the front quadrant), now move them slightly towards you, nearer to your head, together. Experiment with how much pressure to use to lift your head.
Remember, our bodies, in the water will sink - usually the feet goes under first. This is natural. Use this drill to get into balance, and the flutter to provide the fwd propulsion, (and the rotating hands, too).