Refer to figures.
Buffet Margin / Aerodynamic Ceiling
An altitude can be reached where there is only one speed at which the aircraft can fly. In the case of a 1g manoeuvring (i.e. straight level flight), this altitude is called the ‘Aerodynamic Ceiling’. Operating an aircraft at its aerodynamic ceiling would leave no safety margin. In 1g flight the aircraft would be constantly on the point of both low speed stall and high speed buffet/shock stall. Regulations require an aircraft to be operated with a minimum buffet margin of 0.3g.

This is called "coffin corner", and the way to escape it is just to descend. As the aircraft descends, the stalling IAS decreases (that is a compressibility effect of high TAS at high altitude only). The high speed buffet is based on Mach No., and therefore descending at a constant Mach No. would keep you at the high speed buffet boundary, however descending at a constant IAS/CAS would mean the aircraft's Mach No. decreases (see ECTM graph above), increasing the margin from high speed buffet.

Therefore, reducing thrust and descending at a constant IAS will increase the margin from both the low speed buffet and the high speed buffet simultaneously.

Looking a bit closer, descending at a constant TAS would also increase both low and (sometimes) high speed buffet margins (dependent on the atmospheric conditions - i.e. not an inversion or isothermal layer), but we never fly to a constant TAS, and it would not be recommended to do so in this case either. Therefore, constant TAS is not correct (also it doesn't work above the tropopause where this is most likely to happen, anyway).