Yonkyo, "the fourth technique", is one of the basic pinning techniques (katamewaza) of aikido. Within aikido circles, it is regarded with something of an awe, because of the distinct pain it induces.
Not all people are sensitive to the yonkyo technique. Some feel very little of the pain, some don't feel it at all. My former teacher Ichimura sensei said that about one out of five are "immune" to it. In my experience, though, it's far less than that. Through all my years of aikido, I have just come across one or two people who don't feel it at all. All the others feel the pain — more or less — and react to it.
Of course, the aikido student quickly gets used to it, and can withstand the pain, thereby even block the technique. That's of little meaning, but it is still valuable to learn how to deal with such a situation. The yonkyo pressure-pain should be used as a way of breaking uke's balance, to distract him or her. It should not be trusted as a method to control uke by the use of pain.
To create that reaction, I prefer a yonkyo pressure that is sudden and surprising — quite like any atemi, distracting strike. Actually, the yonkyo pressure should be regarded as an atemi, and done in that way.
Aikido students usually have trouble finding the spot on which to apply the pressure, and also how to apply one's grip on uke's wrist. Some precision is needed.
The grip is actually quite the same as the sword grip. The hand positions are twisted inwards, the power of the grip is in the little finger. If you grab uke's wrist in much the same way as you should grab a sword, you are on the right track.
It is the base knuckle of your front hand index finger that is doing the yonkyo pressure. Still, don't let go of the little finger grip of uke's wrist — that will just lessen the effect. Again, it's how you would manoeuver a sword, for example when pressing the tip of the sword down.
The base knuckle of your index finger presses into the flesh of the inside of uke's wrist, and then turns outward to the lower arm bone on the thumb side of uke's arm. It is this turning that causes the pain, not the initial pressure.
Make it tight and sudden, and don't let go of your little finger grip of uke's wrist. Additional pressure is best done by increasing the force of the little finger grip — the index finger knuckle is almost passive.
You get additional effect if your other hand turns uke's wrist the other way, so that your both hands are turning in opposite directions, away from each other. That way, your index finger knuckle and uke's lower arm bone will move towards each other.
Another important detail is to apply the pressure when uke's arm is held so that the upper and the lower arm are approximately at a 90° angle to each other. That makes uke's wrist more sensitive to the technique. If uke's arm is straight, its muscles tend to protect it from the yonkyo.
Don't trust the pain the technique induces. Pain is a relative thing, and some pains are easy to get used to. Use the pain as an atemi, a distracting strike that gets uke out of balance, so that it is easy for you to push uke down.
Shomenuchi
On the above short video clip I do yonkyo on the shomenuchi attack, four times in a row. I don't do the yonkyo pinning at the end, but you can see that on the long new video at the top of this page.
I enter the technique in the same way as I do with ikkyo, that is with a parry that is quite similar to the sword technique ukenagashi. It's not to block the attack, but to get in contact with uke, and to prepare for the circular movement of uke's arm to follow.
In order to have time to apply the yonkyo grip, it is practical to push uke down with the ikkyo movement, before shifting to yonkyo. When uke strives to stand up again, he or she moves right into the yonkyo, and thereby adds to its effectiveness.
Don't apply the pressure with your index finger knuckle before you do the entrance step, or you lose the additional distinction of surprise. If the yonkyo pressure is sudden, it is much more effective than if applied for a long time.
Notice the sword grip on uke's arm, and notice how my hands turn away from each other when applying the yonkyo pressure. Remember to keep a tight grip with your little finger all through.
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The grip is actually quite the same as the sword grip. The hand positions are twisted inwards, the power of the grip is in the little finger. If you grab uke's wrist in much the same way as you should grab a sword, you are on the right track.
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