When a watch has a central seconds hand, the second hand is attached to the same central axis as the hour and minute hand.
All hands of a watch have a certain order of assembly. First the hour hand is mounted, on top the minute hand, and finally the central second hand is fixed. The biggest advantage of central seconds over subsidiary seconds is 'legibility'. Central seconds covers a much larger sweep than small seconds (and is thus often call “sweep seconds”), and it is able to share the minute track on the dial for precision timing over short periods.