In Aikido, we wish to draw an attacker into a partnership, or system. As the attacker adds his/her energy to the system it will inherently effect the system. The speed and verocity of the attack will effect how we have to deal with it. In many cases I like to think of it in a "minus one" way.
The attacker attacks and what he/she gets out of my defense is minus one of the level he/she came in with. This is very much up Bronson's line of ki appropriate for the situation. To think I should effect the system more than this bit is egotisitic and would impose my will on the system, therefore taking it out of balance and it's no longer aikido to me.