I'm not really an authority on this, but I think that perhaps when you become really good (many years of practice
) you will be able to have moved into position and meet the opponents hand in the exact fraction of a second when his arm is extended to the fullest, and from there you can apply the technique. This will also be the time during the attack when the attacker is closest to loosing his ballance.
My teacher emphasizes that the first split-second before the technique, is of outmost importance in Aikido. If you don't get it right - then you can only do the technique if you either muscle it through or if the opponent/partner is moving so slowly that you can go on from there. If he is right it is a question of timing and perhaps this is one of the things that make Aikido so difficult and so fascinating.
Perhaps what makes this difficult is that we tend to think in a sequential way ie: "First I should get away from the attack/block the attack - Secondly I should move into position for applying the technique. Thirdly I should perform the technique. etc.". Perhaps we should instead consider every step as integrated into the technique along with the crucial timing of your reaction to the attack.
Just my thoughts on the matter