First, as I said above, the only options that make sense for this theme are 'gallery' and `empty`.
That said, maybe we are not on the same page as to how this option works. Specifically, it modifies the link which would be to the root page of your site, no parameters, so that it links instead to `page/<custom index page>`. Thus, if you have a breadcrumb the base link will go to the custom page instead of the root page.
If you directly enter the URL of the root, you get the them index.php loaded. In the case of Efferevescence+ this page will vary depending on if 'gallery' has been selected as the custom index page.
A different example of the use of this option is the stopdesign theme. With this theme, the root page is index.php and shows the first six albums. But the custom gallery page is the themes archivepage.php script which is a more or less normal style zenphoto gallery index in that it shows all the albums (paged).
You can observe this behavior in Efferevescence+ by setting the custom indes to 'archive`. Then whenever you click on the album title in the breadcrumbs you will be taken to the archive page. (As I said, not so useful.)