AHAH in Drupal is a feature provided by the Forms API.

• You write the form using the Drupal Forms API.
• You specify what button activates the AHAH behavior.
• You specify what part of the form's HTML is to be replaced by the callback
• You write a callback that generates the replacement HTML
• Drupal manages everything so that when the button or control is used, the callback gets called and the replacement of HTML is done

Although javascript is involved (supplied by Drupal) you don't write any and you don't see any.