By default in impressPages when you switch the language it redirects you to the home page, no matter on what page you are.
What this plugin does is, it looks if there are any translations of current page you are on, and displays only those languages available to switch to.
And instead of redirecting you to the home page, it redirects you to the translation of the current page.
It doing it by comparing the key option (which is added by Alternate hreflang plugin) of pages. When he found two or more pages with the same key, it "synchronizes" them.