Well I'll put my butt on the line on this one...
I've been training kettlebells for about six months now, and while I agree with Mike that they in no way indicate internal martial art practice (if one were to do that one might consider training an internal martial art...) there are some interesting principles at work that might be worth a good debate.
So here goes....
Weight through heels - we tend to keep our weight back and in the heels.
Legs and hips drive the movement - any of the standard kettlebell movements (swings, cleans, snatches) are solely powered by the legs and hips. My kettlebell instructor is forever yelling at me to "stop using your arms" or "let your legs do the work"
"Reverse" Breathing - We do a ton of it to stabilise the torso and help drive the movement.
So that's just a few thoughts. That being said, like anything (look on youtube, for example) there are tons of examples of how not to do it that are super-muscley, with people using way too much weight etc...
Any thoughts on how these principles might apply to aikido practice or where lifting in this fashion diverges and contraindicates IMA practice?
Let me make 2 points, Mike. One, when I meet up with people who claim that they're practicing some aspects of "internal strength", "moving from the hara", etc., it only takes a couple of seconds to see what they've got. And I should note that even a good "external" Chinese art is going to use jin... so people need to broaden their horizons about this whole "internal" discussion.
So I feel for those skills. It's pretty rare that someone has them, out of all the talk about how to do it, special training methods, etc. Or, if someone has some aspects of internal strength, often it is quite limited to linear jin skils; the dantien stuff is non-existent (even though many people are sure that they are "using the dantien"... it turns out to just be their limited interpretation in most cases).
I'm not saying you can't use kettleballs to build up some aspects of internal strength.... I'm just saying you have to know how before you do it. You have to know how to do it before you use a bokken, too. I know from long experience that some of the posters who are "already doing this stuff" will be very embarrassed if they to to show their stuff to someone who really already knows. And I think they may have that suspicion..... hence the lack of people rushing to go compare notes.
O-Sensei used some heavy implements, but I have no doubt that he used them in specialized ways. If you watch his jo-kata where he pokes the jo up to heaven and moves it in a circle, you can see how completely he uses his dantien and his whole-body connection. He didn't get that from ordinary weight-lifting; it's more complicated than that.