Timing

read "Budo" for timing
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http://www.stenudd.com/aikido/ki.htm
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Timing

Because the joining is done with ki at first, and not bodies, there is really no need to wait for uke’s body. The joining of ki can be done as soon as uke has aimed at tori, ie. started a ki flow in that direction. And when uke is committed to the ki flow, so that uke’s body moves along it, then tori can start steering it.
When the steering starts as early as at uke’s initial move, very little body movement is needed, but uke is greatly affected from that moment on.
If you accept and join with uke’s ki already at the moment uke takes aim, it becomes quite difficult for uke to attack at all. It’s as if the target disappeared. Who would ram an open door?

So, there are different timings that can be used in aikido. They correspond to the three training forms gotai, jutai and kinagare.
In gotai ("hard body"), starting from a static position where uke has already applied a grip, what has to be joined with is the ki continually keeping the grip. In jutai ("soft body"), where uke is advancing and the technique begins right before physical contact, tori is using the enhanced flow of ki by which uke is doing the attack. In kinagare ("flowing ki"), the continued training without a marked beginning or end, usually done with more than one attacker, tori needs to join with the ki already when uke takes aim – this way, tori acts so early that it gives the impression of tori initiating, almost as if being the attacker.
The last one, the kinagare timing, is the most advanced one, of course, and is really what the two former timings prepare for. One should cultivate the spirit of this timing, in Japanese sensen no sen, the ability and readiness to join with ki as soon as it comes, before any body movement. Kinagare means flowing ki. Ki should always be flowing.
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http://www.stenudd.com/aikido/ki-exercises.htm
http://www.youtube.com/watch?v=XX4gzo14n14