Ha ha good point you can use FPA for descent too.
Open descent will keep thrust idle and adjust v/s to keep speed be it selected/managed.
DES (managed) will increase/decrease power as necessary to keep on profile and maintain managed speed/meet constraints etc. Speeds will be a band around the managed speed and is a factor of cost index/constraints (ie if high on profile it will use idle power and top band of speed to get an increased descent rate. If low it will use lower speed band and add power).