Gavin,
roll that index finger knuckle on top of the radial nerve and you can create "foot movements"... good pressure will make them dance like nothing you've seen before...

... they seem to get "taller" too...

You can do it both inside and outside the wrist at the same place,only on opposite "presentation" of the radial nerve at the ulner bone. The trick to it is to trap H-4 with your thumb on the "inside position" and with the middle finger on the outside position, and keep the elbow supported. That's the "button" for the "fire-metal" connection and for a good Yonkyo.

The problem with defining martial arts techniques with "points" is that a lot of "points" aren't defined by meridian identification, but they're still painful and work for martial arts applications, which is why I tend to "describe" a technique rather than the points involved.
The inside position of this technique hits right between L-6 and L=7 (you were correct, Eyrie), so giving you either one would be "wrong", but the nerve's still there, so don't get tied up in "point a + point b" equaling "the technique". They're great motivators, just not the great panacea of self defense that replace good solid MA training.

I sometimes call them "pain and suffering incidental to the blowing of the whistle"...

I agree... the points mentioned are merely landmarks (within other landmarks and features) on the body landscape. In any case, capturing and controlling the center thru the elbow is paramount and prior to applying any PP technique. If and when you do, it's pure icing....

BTW, if I've got yonkyo on the top of the wrist (what i call "outside"), I'd be going for TW6,7,8 cluster... rather than LI7

For a nice sensation, take the prescribed grip on the wrist points with one hand and then strike L 6 or LI 7, depending on the orientation of uke's hand in the entry (say shirt grab). Strike obliquely towards the hand. That will make them let go.

There are all kinds of "enhancements" that can be done "after the fact", but as a stand-alone technique, Yonkyo is enough of an "attention-getter" for most people... especially if they aren't used to having it done to them.

http://www.aikiweb.com/gallery/showphoto.php?photo=1328

Ok so if that is how i grab the
hand Then what?

All these numbers/letters your refering to?

First of all, bad technique... the wrist grip shown in the picture might be in the right place, but the support should be at the elbow, not the hand. Someone doing this technique, this way, is either very advanced or very inexperienced...

The letters and numbers are "meridian points" as defined by Chinese medicine. Find you a good acupuncture book and you'll have a road map for these points.

The nuances to Yonkyo are pretty suttle, and it involves simply rolling the knuckles on the "pain points" that make it work, so I won't attempt to detail it here. Any good aikidoka can teach you the technique in about 5 minutes if you're paying attention, and then it's like getting to Carnegie Hall... practice, practice, practice...

Supporting the elbow is one way... locking the elbow through radius and ulna is another... either way locking the entire structure is important.

Actually, you'd be using the top of the index metacarpal at the metacarpophalangeal joint.