D&D 3.5 Overrun avoiding

I have a question regarding Overrun special attack in D&D 3.5. The rule says that, the defender can “simply avoid me“, but it doesn’t clarify if he has to perform any action or roll in order to achieve that. Is that so, that the defender only says “I’m avoiding!” and that’s it? Is there no need for any roll? Does it mean that I can pass through the defender as he wasn’t even there? Does the defender stay all the time on the same square as before while avoiding?

Opponent Avoids? The defender has the option to simply avoid you. If he avoids you, he doesn’t suffer any ill effect and you may keep moving (You can always move through a square occupied by someone who lets you by.) The overrun attempt doesn’t count against your actions this round (except for any movement required to enter the opponent’s square). If your opponent doesn’t avoid you, move to Step 3.

Thanks for your help and sorry if it is so simple.

One more question – why does this table say that Overrun doesn’t provoke AoO?