I believe you are incorrect; driftdown is a procedure where max continuous power and best glide speed will result in a negative climb gradient, ie engine failure above single engine service cieling. Making a drift down calculation will in some cases enable you to overfly terrain which is close to or above you s.e.s.c.
A letdown is sometimes used to describe an instrument approach procedure and does not nescessarily refer to an emergency procedure, and does not have anything to do with a driftdown.