Re: No Balance No Aikido
Maybe an example would help. Consider the case when you are taking ukemi for say iriminage - not so much when someone is doing the low-level yank around, but the more sophisticated version where you feel like you are falling into a void. Taking that kind of ukemi, you have at least three main reflexes at play (the yank around causes other things to happen which can confuse the matter). You have your righting reflex which is getting a bit tricked by a skilled nage - but works to get your shoulders perpendicular to your spine (which should be tilted and lengthened by the nage). You also have the anti-gravity reflex which is why you stick your foot out when you are falling forward, and you have your placing reflex which is why you step back and to angles when you are falling backwards or to the side when your balance is committed to the front leg and you are out of balance. (I'm not doing justice to these, but I hope you get the idea.) The anti-gravity and placing reflexes are complimentary antagonistic (like how your bicep is a bending muscle which is complimentary antagonistic to your tricep which is an extending muscle). Anyway, that is the basic way we move. It has to be a constant state of righting.
In aikido, we get into people's balance through diagonal vectors into the uke's shoulders (and then indirectly their hips). If the nage is a bit more skilled, they can more directly do it from their hips. When the nage is much more skilled, they can do it much more indirectly by leading their partner in such a way that they are naturally unbalancing themselves to continue their attack or maintain their safety.
The line where a human (pretty much any biped who's body conforms to the devine proportion) can be most easily unbalanced (using the least amount of energy) is the line you would imagine drawn through their anus to naval. In shiho nage, the way you are unbalanced up travels up that line, and when you are thrown it travels down that line. The same holds true for a kotegaeshi although it is much more subtle (I know that lots of people perform the lower-level (meaning higher energy required to perform) version where they do a wrist crank to only force loss of balance traveling down the line - but you have to start somewhere!).
In iriminage, I would suggest that the skillful nage will lead you into a a situation (by stepping off line when you are fully commited to the direction of your attack which is I'm sure where Tim was going, but I'll take this further..) where they can get their hand fully attached to the back of your neck so as they twist and extend that arm like the natural turn-over your right hand does when cutting diagonally out with a sword. Since the bottom of their hand is well fitted to the base of uke's neck, and does not break connection with the twist, the uke's righting starts getting lengthen and tilted such that their righting reflex works against them - tricking them into chasing the direction of the technique in an attempt to restore balance. That also works up the line from anus to naval. Of course in iriminage, many things are happening at once. The nage's leading arm (the one between uke's elbow and shoulder) starts retracting a bit like a punch. That leads uke's direction a bit down and in towards where the nage continues to have just been - down that line of balance from naval to anus - which engages both the anti-gravity reflex and the placing reflex - out of synch with the righting reflex. Lastly, since the majority of uke's body mass would be feeling more extended up and out - as compared to the relatively smaller amount of body mass in their attacking arm being lead in and down the nage has to step back away from the direction set by their extending and twisting hand on uke's neck just enough to keep all of the forces in balance and continue to lead. The uke's balancing mechanisms work against each other to the point that the next most natural thing to do is to try to restore balance by depending on an outside source (like gripping more firmly on the rung of a ladder when you and the ladder are falling away from a building). The moment nage feels that connection, the nage simply bends their legs more and uke wonders how they ended up so close to the ground. That line is also why nage has to readjust their direction to be in synch with uke at the bottom of the technique so that they can lead uke's head back up to nage's shoulder as the nage simply steps back and leaves their arms expanding away from their hip (which is performing the back step). I agree that this is more difficult to do with an uke who stops their attacking momentum unnaturally. That's why I'd say no attack - no aikido, but no balance 1/2 of the time is pretty much required for aikido.
All I do these days is work on taking ukemi such that I maintain my safety and honestly of the attack in such a way that I continue to improve my posture and thus my dynamic balance. But, if I were not unbalanced at all while attacking, the attack would have no power unless the nage attacked me first (making them the uke!).