Hi! lets assume you have speed 300kt during your climb phase.The mach number will gradually increase as altitude arises while the speed remanes constant. So, at altitude 28500 speed 300kt is equal to Mach number 0.78. The system switches to the Mach mode (usually in that moment I saw that the magenta target speed triangle temporary, within 1second, jumps down and then up). Then the system keeps the Much number 0.78 while the speed gradually decreases. Also you can use the QRH page 4.16.
Con respecto!