Perfect Technique!
Taken with a rented Nikkor 80-200mm F2.8D that was broken, the aperture blades would only close down to F/5.6, but it was an overcast day so everything worked out. Taken at 200mm F/5.6 for 1/1000sec, everything in the frame balances out, if you don't know competitve swimming, take my word for it, this is perfect butterfly.