If you contrast the photo of the swimmer above, taken from GoSwim.tv's excellent video with the photos below of swimmers I found on various underwater videos on Youtube, you'll see the difference that the correct kick timing makes on hip position and rotation. The swimmer above has timed the downbeat of her kick with the extension of her opposite arm. Her hips are stable in the water, with power clearly coming from the twisting of the torso. The swimmers below are timing their downbeat kick with the arm on the same side of their body, causing an over-rotation of the hips and hence an under-utilization of the power of the torso in their stroke.
Here's an example from another angle. In this photo, you see Olympic medalist Ian Thorpe, the downbeat of his left leg clearly corresponding with the extension of his right arm, his hips very stable and not over-rotating or dipping down in the water.
Versus this swimmer whose left kick is accompanying his left arm stroke instead, thus causing his left hip to dip way down and creating a situation where he can't use the rotation of his torso for power in his stroke.
Kick timing can also be diffficult to work on. Some people get it instinctively, but for others it takes a lot of work. Your timing is often indelibly stamped in your brain-muscle connections and it takes hard work to re-wire that.
For triathletes and lap swimmers trying to improve their body position, their core rotation, hip position, and the power of their stroke, kick timing is an essential skill to work on. I will try to explain proper kick timing here and how I go about helping people achieve it.
It will help if, as people read this, they go back and watch this video with the proper timing as that gives a visual reference to the whole thing:
First, some basics. There are three main types of kicks that we use in the crawl stroke.
#1: Two-beat kick: One beat for each arm stroke. This is used primarily for distance swimming and is the most effective kick you can use with the least amount of effort. A good two-beat kick will keep your hips up for good body position, and will drive the correct core rotation to engage your major core muscle groups to power your stroke. In a two-beat kick, the downward extension of the kicking leg corresponds to driving the gliding arm into its full extension. I like to think of it as drawing an "X" across my body, stabilizing my hips and preventing them from over-rotation. If done right, you will feel this downbeat of the kick powering your arm into that glide. Re-watch the video and look at the pause-point where her leg is at full extension of the downstroke and the opposite arm is at full extension. Although the swimmer in the video is using a six-beat, not a two-beat kick, you can still see what I mean about the timing of the primary downbeat kick.
#2: Six-beat kick: Three beats for each arm stroke. You will still have the downbeat kick in exactly the same place as it was in the two-beat kick, which is to say powering the opposite arm into full extension. The other two beats come in between the downbeats. So it sounds a bit like a waltz, with a feeling of 1, 2, 3, 1, 2, 3. Again, the downbeat kick is the primary driver of the core rotation, and prevents over-rotation of the hips. The additional kicks are in there to provide power for swimming at faster speeds. As you pick up speed, the primary kick stays the same, but the secondary kicks pick up more and more power. Of course, with this power comes increased oxygen consumption, so by the time you reach a full-on six-beat kick, you're going to be sucking up more oxygen than you would want to for distance swimming. The swimmer in the video clip I posted is using a six-beat kick. Many distance swimmers use a modified six-beat kick with a heavy two-beat downstroke and very light secondary kicks.
#3: Kick like hell. This is what you do when you sprint. Unfortunately, all too many triathletes use this option for regular lap swimming, thus eating up their precious oxygen supply. If you are kicking more than 3 beats per arm stroke, you are kicking TOO MUCH. A video analysis will show you how many kicks you're getting in there. Many swimmers kick 4 beats per arm stroke (an 8-beat kick) which has the unfortunate side-effect of having the downbeat kick coincide with the opposite arm half the time (good) but with the same arm the other half of the time (bad). You can spot these swimmers a mile away by the drunken-sailor-like roll that they swim with. One hip consistently dips down while the other stays stable.
HOW TO CHANGE YOUR KICK TIMING:
When I work with swimmers on kick timing, I usually have them start with a pull-buoy. The reason is that if I don't, their regular kicking pattern will quickly re-establish itself. Also, it removes the need to have to kick hard to keep the hips up. I will also note that it helps to have some good basic body-positioning stuff down in order to work on kick timing (head down instead of up, front arm gliding instead of pushing down on the water, kick is efficient without too much knee bend or "bicycling" of the legs).
With a pull buoy, you can start to work on the timing of the downbeat kick. You can work on just one leg at a time. Get a good pulling rhythm going with your arms, and then try to insert just a left leg downbeat kick to coincide with the right arm's extension. Then work on the right leg-left arm combination. Then you can work on both. Then work on eliminating the pull buoy. When the two-beat rhythm clicks, it should feel like each downbeat kick is driving your opposite arm into the glide. Once you get a two-beat kick down, it's not as hard to add in the other 4 kicks to make up a six-beat kick.