If we filled joint last year (2015)' without any amendments, issues, exceptions, etc, can we file separate for this tax year (2016)?

The following statement doesn't make sense to me. "NOTE:  Once you file a joint return, you cannot choose to file separate returns for that year after the due date of the return."

Which year does this statement refer to? The same year as the filed return or the next year?    For example, I can't amend the 2015 return or does it mean, if I filed joint in 2015, I can't file separate for 2016?  If that's the case, how does one ever switch?

Thanks