If you only use one web browser on your computer, you can only log in to a Plone site as a single user.  This makes it difficult to interactively test user access to content when changing permissions, group and role assignments, or tweaking workflows.

Instead, if you use two or more browsers running at the same time, you can log in as a different user in each browser.  Windows includes Internet Explorer as its default browser, and Mac includes Safari.  For a secondary browser, you can install open source browsers that run on Windows, Mac and Linux, such as Firefox and Flock.

For example, I log into the Plone site using my manager account and/or log in to the ZMI using my admin account in Firefox.  Then I start up the Safari browser and log in using a user account I would like to test.  By switching between the two browsers, I can see how this user's access changes as I edit the security of the site.  If I need to verify that certain content or features are not appearing for the public, I can load the site in Flock without logging in.

Permission, group and role changes take effect immediately, so click the 'refresh' button in your secondary browser to see if the test user's access to a particular item has changed in the way you would like.  Note that if you are editing workflows directly, you need to click the 'Update security settings' button at the very bottom of the 'portal_workflow' screen in the ZMI for your changes to be applied to existing content items.