Hi,
The language from browser is being used only if not fully determined from the domain. That means that if you have
www.mydomain.com set for English and fr.mydomain.com set for French, the
www.mydomain.com is detected as English and language is not determined from browser.
Otherwise if
www.mydomain.com redirects to fr.mydomain.com if your browser is in French, you wouldn't be able to switch to English even if you wanted, because it would always redirect.