Swimming Freestyle - The Flutter Kick

The flutter kick is an efficient kick used in the freestyle stroke. A similar kick is used in backstroke in a supine position.

Swimming Video

Here's a nice video that demonstrates the kick:

In the flutter kick the legs are kept parallel and kick up and down in the water.
The feet are in plantar flexion, which means that they point away from the body.
During the downbeat, the knee slightly flexes, and the foot kicks down in a whip-like fashion. The water is pushed down and away with the top of the foot.
During the upbeat, the leg is kept straight and the water is pushed up and away with the bottom of the foot.

The most popular kicking rhythms in the freestyle stroke are the following:

The two-beat kick. You kick two times per complete stroke cycle. This means that you kick once with your right leg as your left arm strokes and once with your left leg as your right arm strokes. This rhythm is ideal for long-distance swimming.
The six-beat kick. You kick six times per complete stroke cycle. This means that you kick right leg/left leg/right leg as your left arm strokes then left leg/right leg/left leg as your right arm strokes. This is the rhythm of the sprinters.

If you are a runner, it might be that your ankles are inflexible and you have difficulty pointing the feet and toes away from the body. In that case, use swimming fins regularly as they will stretch and improve the flexibility of your ankles.
The kick should always be compact and stay within the shadow of the body. If the kick is to overt, the legs will create drag and slow down the swimmer.
Don't kick with straight legs either. The knees should slightly flex during the downward motion.
We have swimming exercises to learn how to flutter kick and balance in a prone position.