As far as I understand, the harder methods use a substantial amount of muscle to train the suit; the softer methods use as little muscle as possible.
Overall great discussion guys.
Anyway to clarify, since Aunkai is considered a 'hard' method, my interpretation is that it is hard because it prioritizes the frame and keeping it in tact (even in awkward positions). This serves as a foundation for 'higher', softer skills. It basically sets up the body to understand aiki/soft skills better. Even Sagawa said that you need strong legs and you need to train the back before you can even do aiki. In any case, Aunkai tanren consists of first training the frame--from there you move on and 'throw away the frame' (Ark's words), and my interpretation of that is that you start finding ways to train softer skills in solo tanren, paired exercises, and freestyle sparring/grappling.
Another example is Judo. There is definitely something internal in Judo in the higher levels. Just look at Mifune. Tim Fong and I were discussing this and he said that Kano would say that Judokas should focus on Tsukuri (setting up a good position for athrow) and then polish up Kake (execution of throw). We think that Kuzushi itself was 'higher' level and perhaps (this is what I think) all the dues you pay focusing on tsukuri would develop a frame from which you can understand the kind of Kuzushi that a guy like Mifune can effortlessly do.
I recently played with some Roppokai guys, had some great fun, and the teacher had aiki. The students too were able to do some aiki, but the difference was in the bodies. The instructor had a frame..the students didn't. It was because during the whole class, there was no tanren, no conditioning, just learning how to manipulate the balance of the uke (not through conventional, vector-manipulating ways like in a lot of aikido practise). And upon reflecting, I don't think the students would be able to perform aiki under pressure, their bodies would go out of wack, and prevent them from executing aiki. The teacher would most probably be able to, but I'm not too sure (this is why it's important we test out with guys who don't play the tori/uke game). Developing muscle through 'hard' approaches (particularly muscle in the legs and in the back) is just a byproduct of setting up a balanced and stable body. We don't develop the muscle to 'train' the suit, at least not directly. Aunkai can now then revert to 'softness'--i.e., we now 'throw away the frame'--and rely on less on the strength of the legs and the back and now try to condition the 'suit' to manipulate the uke. It all depends on how you chart your training progress.