Itten no hi ( 一点 の 秘 ) is the name given to the principle of applying a technique with an emphasis on a single point. That's as clear as mud. What does it mean?
Let's start with Hakko dori as practised in Hakko Ryu. In its simplest form, tori grabs uke's left wrist with his right hand. Uke now frees his hand by bringing it up as if to scratch his left ear. That's the basic technique, but too often it degenerates into a tug of war as tori wrenches his hand free.
Instead of focussing on the wrist where he is being grabbed, tori should concentrate on the tip of his little finger, visualising the finger carving a path through the air. This is itten no hi - concentrating on the one point. It sounds esoteric, but it's quite easy and it does work.
Applying the same principle to nidangi matsuba dori, the emphasis moves away from trying to force the 'Z' shaped lock onto uke's wrist. Instead, the focus is upon the tip of tori's little finger, dropping weight down through that one point.
In a variation on this principle, techniques can be applied by focusing on a single point of uke's, rather than tori's body. In shodan gi, ude osae dori, uke's wrist is pinned to tori's upper arm, a wrist lock executed and then a take down follows. When applying the wrist lock, concentrate on pressing a single knuckle of uke's into your bicep. Typically this will be the index knuckle, though it may change depending on the grip. This knuckle forms the one point through which the rest of your technique is executed.
Why does it work? I don't know. But it does for me. The 'flavour' of the technique seems to get sharper and control over uke more subtle when I try to use this principle. I was first introduced to this idea by Tanaka Sensei of Daiwa Ryu, but the underlying principle seems to travel happily across styles.
It's hard to get the feeling for this subtlety across in a face to face setting on the mat. By trying to describe it in writing on the web I am probably setting myself an impossible task. But I received an email from someone asking about this - so thanks for bearing with it so far. Have a play with the idea and see what works for you, and then let me know!