Originally posted by George S. Ledyard
Every technique has certain points (I call them crossover points) at which a technique may be taken over by the opponent if you leave them the slightest opening. But in order to take advantage of those "cross-over points" you have to be connected and centered at that precise instant. If you are not absolutely locked in with your partner's movement he can make a mistake and you won't be in a position to take advantage of it. Basic Aikido ukemi is designed to prepare you for this next stage.
In all the Aikido technique I have seen, the entire movement is one big crossover point, except perhaps for the last second. This is especially true if uke is not looking for the big score (a clean throw or lock) but just wants to thump you before returning to a neutral position.