This great underwater photo of Phelps during the Fly leg of the 400 IM (click on it to enlarge for closer study) shows several noteworthy things that help make Phelps the best butterflyer in the world.
1) Wide landing and catch. I mentioned in the earlier blog that his stable head position helps concentrate his landing-momentum forward, rather than diving down.
This view, shown an instant after landing adds another piece. Swimmers who are inattentive to the landing tend to bring the hands together in front of the head as they complete recovery. Phelps lands outside his shoulders, with his palms turned out. This landing position helps in three ways: (1) It helps keep his hands near the surface as his chest sinks down. If you could see a side view at this moment, you’d see his chestbone a good 10 to 12 inches below his hands — and his hips highest of all. (2) The wide landing "leaves room" for his chest to sink easily between his arms. The sinking chest creates pressure on his hands and arms for the beginning of his catch. That’s letting gravity help, rather than doing it all with arm muscle. (3) Finally, the wider arm position makes it easier for his shoulder blades to retract (fold toward the spine), which is critical to freeing the lower back to undulate. When the arms are close together in front, it inhibits those movements.

2) Notice the strong stream of nose bubbles. Don’t hold your breath between strokes in Fly. Exchange air constantly, the exhale becomes quite forceful just before your mouth surfaces.

3) Is that kick fully legal or borderline? (Only joking, see my comment below.)

4) How about those lats? Michael’s bulging lat muscles — looking like a Cobra’s hood — show that the muscular load here is far more in his torso than on fatigue-prone arm muscles. At this moment, his intention is most likely to use his hands to "hold his place," rather than to "push water back." When his chest moves over where his hands are anchored, he’ll release and swing forward again.
If you try this yourself, focus on: (a) Use the lightest hold/grip possible (this further saves arm muscles from fatigue), and (b) Make the holding/pulling action vanishingly brief. Return to the front — and a longer, more hydrodynamic body position — quickl