Climb/Descent Rate
Another thing to notice is the change in climb or descent rate when passing from following constant IAS to constant Mach and vice versa. On climb at lower altitudes when following constant IAS the airplanes true airspeed will be increasing as altitude increases. During this portion of the climb, the airplane is both increasing altitude and increasing speed. Excess thrust (that greater than drag) is being used to increase both potential and kinetic energy. Once the airplane has climbed high enough to be into the region where Mach is key, the climb will continue at constant Mach number. During this portion of the climb, true airspeed is decreasing as air temperature decreases and the speed of sound goes down. As a result, while potential energy continues to increase, kinetic energy is actually going down. There should be small, but notable increase in climb rate at the point where the climb transitions from constant IAS to constant Mach number.
The reverse occurs on the descent where the vertical speed at a given thrust setting will be higher when at higher altitude following a constant Mach line down vs. at lower altitude following constant IAS. This effect is what makes speed control going around the corner from Mach to IAS during an emergency descent following Mmo and Vmo somewhat interesting. There is a tendency to overshoot Vmo.
Please excuse the thread drift. It may be helpful to some to think about the physics of the event a little. "Know your equipment better and you are more likely to understand how it behaves - both every day and when the unexpected jumps out to get you."