The working is very simple. You need to choose a "login word" for use as param to add to the site url. This additional param will show the login panel.
Examples of use
Suppose that, for example, the login param word is 'login' you need to make as follow:
In case of SEF active:
In case of SEF not active:
http://www.yoursite.com/index.php?login (if no additional params are already present in the URL)
http://www.yoursite.com/index.php?com_mycomponent¶m1&login (if some additional params are already present in the URL)