i dont believe this is an ex RAF method as its totally stupid! Why would you be descending at below cruise speed?
If anything you will be descending above cruise and watching the speed decay as you level and advance the Throttels back to cruise.
You have two sources of energy in the descent! The potential energy in your engine but more important in the descend you have the energy you are tapping into inherant in the airframe.
If you like you have surplus available in the descent hence the problem is one of going too fast not too slow.
Consider also its bad practice to descend a piston on a closed throttle and this theory makes no sense at all.
Consider also your PAX do they want big burts of power? No they want a smooth flight.
The only time you may want to come down slowly is more in IMC flight where you are entering heavy cloud or thunderstorm or in area of turbulence and are trying to protect the aircraft from structural damage but that is relevant to the cruise as well as descent.
Pace