Here are six important aspects of the kick to focus on:
1. There should be a lot of movement in the ankle; however, from the hip to the ankle the leg should be almost straight. The knees should bend very little. It’s not like pedaling a bike.
2. Use the muscles at the top of the thigh to move the whole leg.
3. The legs and hips should not have side-to-side movement.
4. The kick should be narrow, with ankles relatively close to each other.
5. The kick is not long like a long walking or running stride. The up-down movement should be contained within the hole that your body creates as you move forward in the water. If you kick too wide (up and down), your leg will feel more resistance. Keep your kick out of that higher-resistance water.
6. Ankle flexibility is really important. While your whole leg moves, the foot is like a flipper that provides most of the benefit of the movement. It should feel like you’re trying to flick a shoe off your foot. The key to the kick is coordination and ankle flexibility.