Gives administrators a skeleton key (their own password) to login as any user they'd like.
As an administrator of a WordPress site with many users, it is often useful to login to a user's account to troubleshoot something. Asking for their password is tacky and having to reset it is amateurish. Thankfully, you don't have to do either when you're armed with this plugin. You can login to the site with any user's account by providing admin username followed by a "+" followed by their username and your administrative password. All other user authentication is passed on to WordPress core. Handy, huh?
To login as user joeblow you'd provide:
Username = admin+joeblow Password = [the admin's password]