At DART Swimming we place high value on underwater kick. Senior 1 swimmers work on it every day in practice using a variety of sets and concepts. When I teach the kick to swimmers I teach it using 3 positions. These 3 positions simplify the movement and I find this method to be a great way to break the kick down into ‘chunk-able’ bits that can be learned by just about any swimmer. A similar article I wrote for swimswam.com was published here: https://swimswam.com/shout-from-the-stands-teaching-underwater-butterfly-kick-positions/
This is the starting position for every kick. The body should be in a straight line from fingers to toes and should be either flat or headed toward the surface.
The hips and knees simultaneously flex to move the swimmer’s lower body from position 1 to position 2. Both joints must flex in order to maximize force production. This is performed by moving knees forward
This is the end of the propulsive phase of the kick. The swimmers pushes the water as hard as possible extending the knees. The feet should be deep.