Without this module, if during the processing of an AHAH callback, css files are added using drupal_add_css() but those files aren't already loaded on the base page from which the AHAH request originated, then those files don't get loaded. This is an unfortunate limitation of Drupal's AHAH system. This module addresses that limitation. However, to take advantage of this, a module implementing an AHAH callback needs to send the response through theme('ahah_response') as provided by the ahah_response module.
If you're interested in this module, you may also be interested in the ahah_script_ensurer module.