The FormsAuthentication.RedirectToLoginPage(string) method redirects the browser to the FormsAuthentication.LoginUrl and includes the extraQueryString value as the System.Management.ManagementQuery.QueryString for the redirected URL.
The FormsAuthentication.RedirectToLoginPage(string) method does not clear the forms-authentication cookie. You can use the FormsAuthentication.RedirectToLoginPage(string) method in conjunction with the FormsAuthentication.SignOut method to log one user out and allow a different user to log in.