Physique
Phelps is anatomically perfect for the demands of the butterfly stroke.
His abnormally long, triangular torso and (relatively) short legs provide
minimum drag in the water, while his enormous 6'7 wingspan enables him to
cover more distance with each stroke than his competitors.
Extra-large feet (size fourteen) and hands propel him through the water.
Technique
Physical attributes - notably double-jointed ankles and elbows - also assist
Phelps in producing a distinctive swim style.
His dolphin kick, a crucial move which sees the standard leg kick replaced
with an explosive whip-like motion, is infamous.
To do it well you need strong legs and flexible joints. The American's ankles
bend around 15 degrees further than his competitors - allowing his feet to
act like flippers.
When timed correctly, this technique can be worth half a body-length's
advantage on every turn.
Phelps - like Mark Spitz - also breaks all the rules by breathing after every
stroke. It is very difficult to achieve this without slowing motion through
the water.
Other swimmers follow conventional wisdom, breathing only on every other
stroke.