All you need to use this feature is to decide on which of the domains should be considered primary for your site. Then simply enter the chosen domain version in the settings screen of the Site app in your Webasyst backend and save the settings.
Done! No extra setup is needed for the redirect to start working. Now, if a visitor tries to open your site on the non-primary domain version, i. e. the one not specified in your site settings, he or she will be automatically redirected to the primary domain version, i.e. the one you have specified in the site settings.