I attended a seminar at Northeast Aikikai last weekend featuring Keith Moore of Chicago. Keith sensei had some interesting things to say about timing in Aikido. He illustrated them with a bokken, but the concepts apply to unarmed situations as well. Three kinds of timing are defined: go-no-sen, sen-no-sen, and sen-sen-no-sen.
Reactive timing limits your defensive options. In reacting to an attack, aikido defense starts later than it might. Time is lost recognizing the fact of an attack, determining what kind of attack, getting over the fear of threat, etc. The student loses time in which the attack can be influenced.
Blending with the attack is possible because the student starts to recognized signals in uke's body that an attack is starting. At this stage, most of these clues are large and obvious. Uke may take a step forward, raises an arm, reaches to grab, etc. Training allows the student to rapidly adjust his own body motion to match and blend with the attack. This effectively happens at the same time as the attack.
In weapons training, simultaneous timing leads to the ai-uchi situation, where both participants attack at the same time. Depending on how this is done, it can lead to a mutual hit, or one person missing while the other hits. It reflects a total commitment on both sides to the attack regardless of the outcome. This is sometimes called "embracing the void".
Anticipatory time gives the student still more options in how to deal with a potentially violent situation. There is time to reason with the attacker, move out of range, distract the opponent, invite a particular attack, and other, less violent, options. A master of Aikido will be able to adjust his stance and ki such that uke is left without an opening to attack. With a deep understanding of both the physiology and psychology of an attack, it is possible to create a situation in which the attack is very difficult or impossible.
Mr. Moore also mentioned a situation in which two opponents come together, prepare for battle and then disenage because one acknowledges a superior, or both acknowledge equality. I belive that sensei referred to this as ai-noke. It is the logical conclusion of anticipatory timing because both sides of the conflict see deeply into his opponent, observing all of the signs, including skill level.