Let's just clear things up. If you pull 'open descent' above crossover, the aircraft will maintain the Mach number say 0.77. At around crossover or when the Mach/button is pressed, it will change to CAS at the current selected Mach/CAS. As you descend, the overspeed protection and indication will move from MMO to VMO...and gradually move up the CAS scale towards its stated VMO.
Assuming that an individual wants to expedite descent, or 'kinetic energy', in open descent it would require the selected speed to wound up towards VMO. I.e. A speed change