Wikipedia: that well-known peer-reviewed source of truth.According to Wikipedia (WITH references!):
Flutter kick is the alternating up and down motion of the legs, either from the hips or as the more restricted movement of the modified flutter kick, and is the most frequently used finning technique. Flutter kick is easy to learn and is the technique most commonly taught to learner divers, but is often applied badly, where the fin is moved in a "pedalling motion", which reduces the effort required to move the feet, at the cost of making the effort largely ineffective for propulsion. Almost all types of fin are at least reasonably effective for flutter kick, with the exception of the monofin.
For maximum power from the flutter kick the full length of the leg from the hip is used, as kicking from the hips uses the largest muscle groups Having one stronger or leading leg tends to propel the diver in a curve, particularly if there is no visual feedback.
Flutter kick is effective for acceleration and sustained speed, particularly over moderate to long distances. It is a strong technique and can produce high thrust, so it is effective when swimming against a current. Sustained moderate to high speeds increase gas consumption due to high energy output.
Forward movement through the water has been used as a substitute for neutral buoyancy, particularly before buoyancy compensators became available, and still is used for this purpose. The flutter kick has a tendency to kick up silt from the bottom from downwash, but is good for avoiding contact with a nearby vertical surface, as when swimming along a wall.
Must get in there to edit it to include the bicycle kick variant best used by Buddha buoyancy proponents.