Whether fast or slow your opponent always moves in a particular rhythm. The perception of the gap in the rhythm means to see through the gap in your opponent’s rhythm, or to cause it and then use your own effective rhythm to attack him.
If an opponent has a slow spirit, you should at close proximity, without moving your body, without showing the beginning of your sword stroke, with a free spirit of the void strike quickly. This rhythm is Ip-pyoshi.
If an opponent is impatient, you should imply with your body and spirit that you wish to attack him immediately and when he responds with an attack, you should strike him after his movement. This rhythm is called Ni-no-koshi.
Munen-muso means: always holding your body prepared to strike, but at the same time holding back your spirit and your sword, as soon as you see an opening in the spirit of your opponent you should strike him strongly with a free spirit of the void. This rhythm is called Munen-muso.
There is also Okure-byoshi. This rhythm means that, when your opponent tries to block or parry your sword, you should, with quite a slow movement keeping your spirit stagnated in it, cause the intention of your opponent to fail and take advantage of the consequent opening to strike him.
This should be carefully practiced.
Musashi talks about rhythm and the gap in this article. Remember that Ma is the distance between you and your opponent as you are trying to cut each other. This distance can be in space or in time, here we talk mostly of time, but distance is of course involved, it must be.
Musashi tells us that our opponent moves in a certain rhythm, a certain speed and timing which we can detect. More specifically, we can detect the gaps in that rhythm, the places where an opening occurs. These might be as he lifts his sword, just before he cuts down or just after he cuts. Elsewhere Musashi warns us of our own telling rhythms, and against trying some tactic more than once, lest our opponent use it against us by catching our timing. You can catch the opponent’s timing by observation or you can trigger it. Musashi writes, in the Go Rin no Sho, of infecting an opponent with your own timing and you can try this out yourself when doing kata. See if you can speed up or slow down your partner by your own speed, by the sound of your kiai, or by the pressing or releasing of pressure by your body.
Ippyoshi is the timing of “at once”. From a distance that allows you to strike, do so without warning or preparatory movement when you find you are up against a slow opponent. This is the “high school kendo” movement of simply getting in there first.
Ni no koshi is what you might use against that high school kid with the fast twitch muscles. Shift toward him slightly, apply a bit of seme and watch that sword whip down in front of you, then as it does, hit him. Remember the articles about treading down the sword and moving the invisible shadow, trigger an attack and respond to it.
Munen Muso is to hold ready to strike, perhaps in jodan but from any appropriate position, and then watching with your spirit, holding your sword ready. Now, without thinking about it, just allow yourself to cut when you see an opening. The opening will appear, it is an expert indeed that does not have gaps, just wait and strike the instant it appears. If you are thinking and planning you will miss your chance. This is striking from the void.
Okure-byoshi is the fourth timing that Musashi mentions, and again, it is a triggering tactic. This time you don’t trigger an attack, but a defence and then strike into the opening created. I was once delighted by watching Ohmi sensei in a kendo match set up and defeat his opponent by attacking the head three times. The first two the other fellow blocked easily but on the third Ohmi sensei came in to attack quite slowly, up went the block in the same exact way, down came the do cut for a nice clean point. This was what Musashi called Okure-byoshi.
How do we train to catch the rhythms of our opponent? One of the easiest is to match the timing of our sensei and of our fellow students. When teaching an iai class with a mix of students I very much like to see the group move in unison. As the beginners try to remember which way to step, which way to turn and cut, the presence of the crowd around them will teach them the movements and timing much faster than I could ever explain and demonstrate. The seniors, meanwhile, are learning to catch the timing of those around them.
At the same time, in order to avoid creating peculiar and identifiable timings in our own practice, we should also follow the crowd, or sensei in their timings. If we constantly move at our own idea of correct timing (creating a room full of chaos as swords rise and fall in different directions, scaring the bejeepers ouot of sensei) we will create a fixed rhythm in ourselves. One that an opponent can then break.
If we become familiar with many timings, fast and slow, we become hard to predict and we develop the ability to fight many different timings.