h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

If you just avoid the attack by removing yoursefl from the point you were a split moment before, and the attacker is good, the attack will just continue from another direction. Aikido is much much more sophisticated than avoiding and then coming back and throwing. Aikido is about total control over the attack from the beginning to the end of it. Before the attack gets to its planned climax, it is taken under control, diverted, and often prolonged beyond the point where is was supposed to stop.

About why the attacker doesn't get up and attack again - this is often because he is by the moment dead or desperately needing medical help or unconscious, depending on what you wanted to achieve. Imagine the Earth being a trained boxer's fist and you'll understand how offensive it may be to be thrown at head head on. The wide spread opinion that aikido is a soft and humane art is very wrong. Just remember that most of the techniques come from ju-jutsu and fencing, which are not, in any way, humane.