This is a very nice technique. Seichin used to be my favorite kata in the entire Uechi system. I am still meticulously dissecting it, and I have come up with literally a dozen applications for one of the movements in this form.
Some (like Evan Pantazi) like to come up with very specific applications in kata, and feel that there are really only one or two that the author meant. I've always felt that the Uechi system of karate was a bit different. It is so parsimonious in its choreography, but so many different things come out of each movement.
With that approach in mind, I have always felt that this particular movement could be thought of in several ways. The wrist movement represents a diversion from The Big Three (tm). You will not find this movement anywhere in Sanchin, Seisan, or Sanseiryu. However you WILL find it in the superinpei form that I learned indirectly from Shushiwa's nephew. In fact, it is done over and over and over again in the form. It aways happens at the end of a sequence. I think that it is perfectly plausible (though I cannot prove it) that this movement represents some of what Kanbun taught his son but was never passed on as part of a form per se. If we can believe the stories about Kanbun and Superinpei (Breyette sensei tells me that Toyama sensei confirmed that Kanbun was shown such a form), then this may give a tantalizing view of the movement in an earlier context.
IN GENERAL....the first half of this wrist technique can be block or attack...or BOTH. As a block, you are using the forearm and can do this in spite of the fact that you may have just damaged your hand in the fight. As an attack, you are striking with the hard tip of the radius.
IN GENERAL....the second half of the movement hints at the southern Chinese origin of the system. One can appreciate this movement more if one does chi sao of the Wing Chun system. It's commonly referred to as sticky hands. Basically the "pretty" rotation of the hand before the grab and pull is a way of maintaining contact with the opponent as one transitions from block/attack to grab. If you ever evolve in your martial training to practicing bo kata, you will recognize how common this principle is applied when doing hand changes. An experienced practitioner of kobudo does not grab his weapon in a static manner. Instead he/she skillfully transitions quite fluidly from one grip to the next.
I will give you a very specific interpretation that "jumped out" at me as I was practicing the superinpei. In this form, one repeatedly turns the body while doing "crane hand" with the left, and a lateral wrist movement with the right. And it is ALWAYS at the end of a sequence. At first I thought it was just a way to turn toward a NEW opponent. Then one day I was reading a book of Dillman demonstrating a kyusho application of a common Okinawan kata and BINGO!!!! I will just tell you what I am doing. I don't feel comfortable with all the Chinese accupuncture logic of the techniques; I'll let Evan chime in if he wants. Me....I like my western medicine.
Basically assume you have been able to grab onto a person's wrist with the one (stationary) hand. There is a way to hold the wrist where you are really clamping down near the condyles of the radius and the ulna. If you clamp down and rotate, you can give quite a sensation to your opponent. The most notable one is the unpleasant stimulation you give to the radial nerve. Now WHILE you are stimulating the wrist in this fashion, do a wrist strike right underneath the ear of your opponent. You will simultaneously be pulling on that wrist and striking to the side of the head that is flying towards you. With all the action happening at once, the wrist sends a shock wave in a slightly upward angle into the brain. If you do it right, you are virtually guaranteed to knock your partner out.
Hope this helps.