This proposition has two parts. Part 1: The Head is always… This defines the Head of a Partzuf. Part 2: …that is, through the mystery… This explains the above premise.
Part 1: The Head is always the beginning of the entire government of the Partzuf… The Head itself includes everything that exists in the entire Partzuf – but in the form of a beginning (ראשית, reishit). This is why it is called the Head (ראש, rosh), while the function of the Head is completed through the rest of the Partzuf (see Opening 90).