I read this article just recenlty and really thought it was a great explanation to the importance of the kick especially the 6 beat kick for most open water swimmers and triathletes.
While I have been swim coaching I encourage swimmers to push toward a 6 beat kick, but often as they do this they say "it is unsustainable" for them. Moving away from a slower kick to a 6 beat kick doesn't mean that you have to kick harder it becomes more of a flutter kick than a hard core sprinting type kick (sprint kick which is generally unsustainable for most tri or open water swim race distances).
This flutter kick at a consistent level creates balance in the stroke and consistent forward propulsion. A poorly executed 2 beat kick I believe usually creates excess drag through the legs working like large stabalises to the body movement and it is usually when you see a hammer kicking action or a very spread eagled style kick to counter balance the body"sinking" or an unstable body rotation.
If there is one skill that most differentiates the fast swimmers from the not-so-fast swimmers, it would be the strength of the kick. As a triathlete, one of the biggest dilemmas, given the limited amount of swim training time you have, is how much time and effort you should spend trying to improve your kick. I believe focusing on the legs is one of the best ways to improve as a swimmer.
The kick plays a huge role in overall speed because it provides four important functions:
1. Depending on the strength and fitness level of the legs and the ankle flexibility, it creates propulsion.
2. Because the kick is not symmetrical, it creates lift—the downward phase of the kick is more powerful than the upward.
3. Since the most powerful of the down kicks coincides with the end of the underwater pull, it helps stabilize the counter-rotation of the body.
4. In a six-beat kick, there is virtually no recovery time, generating a steadier propulsive force. (Only use a two-beat kick if you do not choose to focus on your legs to improve your swim time.)
Note: Even though a wetsuit handles one of the primary functions (creating lift) during a triathlon, a good kicker can still benefit from the other three functions.