As promised, here is that video on whip dynamics. Watch the whole thing, or start at 5:45 for the gist of it. The speed of sound is indeed (well done Alex and Ben!) 343m/sec or 1,234.8km/h (which for all you old timers, air force or navy people is 667 knots).
The similarity with the shinai, as I mentioned at training this morning, is the 'unrolling' of the whip and how this causes the speed of the tip to increase exponentially. Our aim with the shinai is not to interfere with the whip-like potential of the shinai, by having the most rational and efficient cutting action possible. We should try to imitate the kinetic wave of the whip's motion through the way we use our body, recreating this unrolling by starting with the shoulders then continuing through our upper arms, elbows, forearms, wrists, palms and fingers.
As you can see in the slow motion video of the whip with the taped segments, the unrolling movement is characterised by each section of the whip coming to rest at the horizontal after the wave has passed through. In the same way, each segment of your arms should stop at the horizontal in a sequence from shoulder to fingers.
Lastly, it is clear from watching this video that the greatest acceleration happens in the last few centimetres of the whip. In a sense this is why it takes us by surprise; most of the perceptible movement is quite slow. The same is true of the shinai. The movement of the shoulders and upper arms is not great, either in speed or distance. The speed of the kensen is of course amplified by the leverage afforded by the length of the shinai even at this early stage. But it is not until we use our wrists, palms and fingers in a co-ordinated sequence that the kensen really accelerates.
"Wrists, palms and fingers in a co-ordinated sequence" is just a long-hand way of saying 'tenouchi'!