Wow... go to sleep and a flurry of activity occurs....
Great posts Shaun (esp. #73) ! But I think I lost you on the time/space relativity thing...
(Time to brush up on Quantum Physics?)
Sorry, Chris... what I should have said is:
...it's really locking (or pinning) their "center" through the anatomical structures that's the real "key"
As Shaun explained... there are many centers. From a simple mechanical perspective, when you move the point of the fulcrum and/or change the length of the lever, the center moves. But we're not doing jujitsu, although understanding how it works in jujitsu can be helpful.
The difference in pinning someone using aiki and plain old jujitsu is the "quality" (for want of a better word?) of the pin. As Shaun mentioned, it's not primarily pain compliance, but being pinned in such a way that uke finds it hard to move/struggle because it feels like they're stuck - not because there is pain centered on the joint(s) which is(are) being pinned/locked. It's the same reason how you can "pin" someone with one finger on the elbow and knee, and they can't go anywhere or get up.
As Shaun mentioned, joints can be opened/closed, or locked/unlocked. You can "lock" a person from their fingers to the toes (yubitori-waza is a good way to feel how this works - without the jujitsu type pain compliance). It gets really interesting when you "open" uke's joints and establish a path to their center, and they don't know what the heck you're doing and why they can't do any thing. And if uke closes a joint off (as in "resist"), the anticipated center of the movement changes. The aim of aikido is to establish harmony (whatever that means?), opening that which is closed, closing that which is opened, to establish a path to uke's "heart" and create movement from the center (any number of the multitude of centers or center of the center). Sometimes you have to unblock what is blocked, or fill what is empty, or empty that which is filled.
It requires a distinctly different spiritual mindset to find the "blockages" in uke's structure. To this end, tori is merely a vehicle for uke to experience "opening" and "closing". Very, very different to jujitsu...
Hmmm.... pins and locks as misogi
? But I'm getting ahead of myself.... and I think we lost Darren along the way back there.