What is the key component in the second step of butterfly progressions?

The key component in the second step of butterfly progressions is the dolphin kick, often referred to colloquially as "super glue" due to its foundational role in the stroke. The dolphin kick is essential because it generates the thrust needed to propel the body through the water. This kick helps to maintain body position and efficiency in movement, enabling swimmers to develop the correct body undulation that is crucial for executing the butterfly stroke effectively.

Focusing on the dolphin kick allows swimmers to master their core engagement and timing, which are essential for progressing to more complex elements of the stroke, such as the arm movement and breathing patterns. By solidifying this aspect of the swim technique in the early stages, swimmers can build a strong foundation, facilitating smoother transitions into the arms and coordination required for a full butterfly stroke.

In contrast, the other options relate to different stages or aspects of the butterfly stroke and lack the foundational focus that the dolphin kick embodies in the early progressions.