Advanced editing mode in SharePoint Designer 2010

Ever wondered what the difference is between editing page layouts in default mode or advanced mode in SharePoint Designer 2010?

Editing page layouts in default mode is possible when you include a Web Part Zone in your page layout. Editing page layouts in advanced mode allows you to customize anything on the page layout page. Advanced mode is restricted to Site Collection Administrators.

If you find yourself editing a page where certain parts are locked for editing, like in the following example:

simply go into advanced mode, by clicking the button in the ribbon (Home tab, Editing section):

you will now be able to edit all code on the page layout page: