Rama

Make the initial temperature change slowly. We want to start the temperatures going down early. Once you establish the downward trend in temperatures, there is far less likelihood of causing engine damage by fast cooling.

There are three ways in which you are going to cool the engine:

1. Reduction of power.
2. Increase in airspeed.
3. Enrichening the mixture.

The wost thing you can do is to do all three of them at once. Start the initial cooling trend by lowering the nose by keeping the power and mixture the same. Try to make small changes in RPM and manifold pressure. Limit yourself to 1 or 2 inches of manifold pressure reduction at a time. To prevent rapid turbocharger cool down during descent, the desired EGT is 1350F.

Every flight, you should anticipate a rapid descent and start a cool-down before the let-down. Reduce power or open the cowl flaps part way to start reducing the temperature of the engine. When you have to do the fast descent, you can close the cowl flaps and be less concerned since you have already reduced engine temperatures.

You do not want to do a long let-down with the prop driving the engine. This causes ring flutter and broken piston rings. High RPMs increase the up-and-down inertia load on the rings in the piston grooves. Cylinder pressures help to keep the rings seated to the bottom of the ring groove. Reducing RPM to low cruise will be beneficial by reducing ring inertia and increasing cylinder pressures. Of course, at some time (as on final) the prop will be driving the engine and this is okay for a short time. But a long descent at high RPM and low manifold pressure is hard on the piston rings.
From The Skyranch Engineering Manual. Lycoming says similar things.
The myth is that you can run a piston engine at full/climb power for 20 minutes and then descend at a low power setting at a high airspeed all day long day in and day out with no ill effects.