Thank you all for the explanation.
Thus on descent when flying at Constant Mach, IAS increases knot by knot due to the higher compressibility of air at lower levels, Rate of Descent increases as A/C wants to pitch down to increase IAS. When A/C reaches max TAS, change over altitude happens and the A/C switches over to IAS decreasing the ROD?
Do correct me if I'm wrong as what I've written is base on my understanding of all of your generous inputs.