Regan Penaluna in Nautilus Magazine:
[Misty] Hyman came of age as a world-class swimmer during the underwater revolution. “I was 13 when I started staying under water longer than is typical,” she says, explaining she could go 30 meters without breathing. “I found I could be faster under water than at the surface.” Most swimmers were using the dolphin kick to propel themselves underwater, but Hyman’s coach, Bob Gillet, wanted to experiment. In 1995 he came across a study in Scientific American about how tuna were able to swim at almost 50 mph, where dolphins top out around 25 mph. The study found that the flick of a fish tail generated more efficient thrust than that of a marine mammal tail. Gillet wondered whether the dolphin kick might be more powerful on its side, so the undulations were horizontal, like those of a fish.
One cool December day in Phoenix in 1995, Gillet put it to the test. Hyman showed up for practice at Gillet’s outdoor pool, and he asked her to try it. “In the most respectful way, I called him a mad scientist,” she says. Her first attempts were awkward, and she ended up three lanes over from where she started. But she got better, and soon she was cutting through the water like an eel. She was going faster than she did with the dolphin kick. Faster than she had ever swum before. This gave Gillet another idea.
They went to the local country club pool, where the lighting was brighter and Gillet could walk out to the edge of a diving board to capture video. They took a long, thin rubber tube, fastened it to Hyman’s wrist, ran it down the length of one side of her body, and fastened the other end to her ankle. Then they filled the tube with store-bought food dye, and Hyman corked the tube with her thumb. She jumped into the pool, released her thumb, and took off as Gillet filmed. What they saw in the footage afterward astonished them. The dye swirled out to reveal huge vortices after each of her horizontal kicks. Gillet suspected that these miniature whirlpools, reaching 4 feet in diameter, propelled her forward. He also thought it was possible that when Hyman did the dolphin kick facedown, the bottom of the pool and the surface of the water interfered with these vortices and slowed her down.
More here.