Dolphin kick

The kick used in butterfly is called a dolphin kick because the legs move as one unit, like the tail of the dolphin. This kick is also used underwater during the starts and turns of butterfly, freestyle, and backstroke (and in the fly-back and breast-free IM transition turns) One dolphin kick consists of two parts: the upbeat and the downbeat and swimmers execute two dolphin kicks during each stroke cycle.

Upbeat

The upbeat of the dolphin kick begins as the downbeat of the previous kick is nearing completion. That downbeat starts a rebound reaction that pushes the thighs upward to initiate the upbeat. Continued extension of the hips keeps the legs sweeping upward until they pass above the body, where the upbeat ends and the next downbeat begins. Most of the upbeat should be made with the legs extended. The lower legs and feet should be relaxed and passive so that the pressure of the water pushing down from above keeps them extended. Water pressure also pushes the feet into a natural position midway between extension and flexion. Swimmers should gently flex the legs at the knees in preparation for the next downbeat just as the feet pass above the hips. The upbeat can be propulsive but not in the extend the downbeat is. It's primary role is to set up the downbeat.

Downbeat

The downbeat a whip-like motion that begins with flexion at the hips and continues with extension at the knees. The downbeat begins when swimmers press down with the thighs as the feet pass above the body during the preceding upbeat. The water pressure, which is now pushing up from underneath, causes the lower legs to flex upward even further. Water pressure also pushes the feet up and in to an extended and pigeon-toed position, even as the thighs are pressing down. The thighs do not press down for very long. Once they start down, their movement initiates a wave-like extension that travels down the thighs to the lower legs, which are forcefully extended at the knees. The downbeat of each kick ends when the legs are completely extended and the feet are slightly below the trunk.  

Swimmers who have better flexibility in the ankles, and lower back and they also have hyperextended knees can gain more from their dolphin kicks as their feet can remain in a position to press back against the water throughout the majority of the downbeat unlike other swimmers. Apart from these physical traits, swimmers should have strong hips and core to produce a strong dolphin kick. Strong hips and core provide a stable platform for the legs to kick off of and provide a better control of the direction of the kick.

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