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How do cetaceans swim with vertical tail strokes?

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Illustration showing various cetaceans, including baleen whales like the blue whale, fin whale, humpback whale, gray whale, sei whale, northern right whale, and bowhead whale, as well as toothed whales such as the sperm whale, narwhal, beluga, Baird's beaked whale, killer whale, dolphins, and porpoises. Each species is labeled with its scientific name and typical or maximum length, with a size scale in the corner for reference. — cetacean Whales (order Cetacea).

Cetaceans swim with vertical tail strokes because they evolved from four-legged terrestrial mammals that moved their limbs in a vertical plane, rather than the horizontal plane used by fish or crocodiles.1

This movement is powered by large epaxial and hypaxial muscles running along the spine. These muscles drive the tail up and down, which in turn moves the horizontal flukes to provide propulsion. During this process, the body acts like a spring, flexing at a point between the dorsal fin and the anus, while the thorax and abdomen remain relatively inflexible. This biomechanical adaptation allows these animals to travel through the water with high efficiency.21