At Nanseikan we often use the term kikentaiitchi (気剣体一致) to refer to the timing of the right foot with the moment of striking the opponent with the sword. It is one of the most difficult aspects of kendo technique. Many people spend a long time trying to achieve it. I spend a lot of time trying to explain it, but I never feel like I can do so in words. It is something that just needs to be practiced with the body to be able to understand.
However there are a couple of important points worth thinking about when it comes to KKTI.
Suburi timing vs sho-men uchi timing
One of the difficulties comes from the the fact that we teach beginners one kind of timing when they first learn suburi, then expect them to learn a completely different and contradictory timing when they start to hit their opponent.
Suburi timing involves lifting the hands for the upswing as soon as the front foot slides forward, with the downswing coming as the back foot slides into place.
Sho-men uchi on the other hand requires no movement of the feet or body with the upswing, but then a co-ordinated lunge and step forward with the whole body when you strike. The stamping step is called fumi-komi ashi. The action of kicking off with the back foot to drive your body forwards is called fumi-kiri dosa.
Being able to do both these two conflicting timings whenever you choose is quite difficult but will come with practice.
No, I don't mean "waiting"! I mean how you weight each foot when you do kendo. Many people say you should have your weight a bit towards your back foot, say 40% front, 60% back because you cannot move your front foot when you weight is resting on it. That's wrong for two reasons. Firstly you can't move your front foot when there is any weight resting on it. Secondly it means you are starting by leaning backwards with your body, which doesn't fit with the kendo mindset of always attacking forwards.
There is a very clever way of getting around this problem of "un-weighting" the front foot without having to lean backwards first. It is clever because it also helps us with achieving kikentaiitchi, and even many other shikake waza such as harai waza, ose waza and makiotoshi waza.
You fall forwards.
With your weight evenly on both feet, apply a small amount of extra pressure with your left heel (which as we know is always slightly raised in kamae).
This will cause your centre-of-gravity to move forwards. In other words you will start to lose your balance because you body is moving forwards but your feet are staying still. At the same time as you apply this pressure with your left heel, push your left hand forwards. This starts your backswing.
Just as you reach the point where you are really about to fall over, your swing should be at its apex (highest point, which is 45 degrees above your head for kihon waza). At this point your feet should not have moved, which means your shoulders will be closer to your opponent than your front foot, not a position you can stay in!
So before you fall flat on your face, kick off with your left foot.
At the same time, bring your sword down to cut the target area. Allow the sole of your right foot to land flat on the floor, which is the fumi-komi ashi.
Then bring your left foot up and continue to follow through using okuri ashi.
When the cut and the right foot are in unison, this is the basic achievement of kikentaiitchi.