George S. Ledyard
My current working definition for "aiki" is using your partner / opponent's sensory inputs (including the intuition or 6th sense) to move their mind so they they move their bodies. Hence the statement made by one "aiki" teacher (I don't precisely remember which one said it) that "If you understand what was just done to you, it wasn't aiki". If you read interviews with folks who took ukemi from or tested the Founder, uniformly they don't use "active" verbs. It's never "he threw me" or "he did this or that..." It always seemed to be phrased "And I found myself flying across the room..." There was always this sense of the inexplicable...
Steering clear of the evil "I" word, it starts with how you organize your own body and how the intent can give you structure rather than the conventional use of muscular tension. This is usually trained starting with direct physical contact to provide immediate and increasingly subtle feedback. Later on, you start to realize that your partner / opponent begins to be affected long before he actually touches you. The mere act of organizing your self into a coherent and balanced whole has observable, tangible effect on a partner / opponent starting when he puts his attention on you. In fact, the REALLY good guys can effect you even when you have not consciously put your attention on them. In effect, they "hack your system". So, the result is that the act of using ones intent to organize ones structure eventually leads to the ability to use that intent to mess around with the other guy's structure.
You bring up a good point - how you are organized does affect the other guy's perception of you. I think back to descriptions I've heard of sword duels where one opponent admits defeat because the other "has no openings".