Drift down speed is used when, for a variety of reasons, either a powerplant is shut down for precautionary reasons or fails enroute, necessitating continued flight at a lower altitude.
The speed selected is usually that to obtain the most nautical air miles, depending on terrain enroute.
Normally engine thrust is increased on the remaining engine(s) to compensate for the failure. In addition, a lower cruising speed is required.
But not always....for example, Howard Hughes operated a Lockheed Constellation in cruise flight (at a lower altitude than normal) using only three engines at cruise power, and obtained a slightly faster cruise speed than with four operating engines.
Oddly enough, this was not possible with Douglas airliners of the time (DC6 for example).
The Constellation was a very clean design, especially the 1649A.