David Orange wrote:
But since aiki comes from kenjutsu originally, how does that work sword-against-sword? And how does it explain aikinage?
Actually that is a fundamental concept in kendo. Via both shinai to shinai contact and taiatari, you feel your opponents intent, that is when they are going to attack and you use that to maintain center (in a manner similar to feeling one recieves when performing push hands). This is best done through the support structure of the body rather than relying on the muscles of the arms to keep the sword rigid, as your opponent can use that focus against you as you can generate move power to move your opponents shinai or simply strike via the weight of the body than just the muscle/weight of the arm. This results in your oppoents shinai moving out of the way so that a proper point can be scored, or in unbalancing your opponent so that a point can be scored because you have taken advantage of where your opponents sword and or body was weak.
The concept of seme covers this, and infact you need not make contact with your opponent to have the same effect upon them.