This is going round and round in circles
Think energy management airframe and engine.
Going downhill
We are tapping into the potential energy in the airframe as well as from the engine meaning the aircraft will accelerate! steepen that approach and it will accelerate even further.
I always like to think of two throttles the engine one and the elevator so we are managing energy.
We also have various chunks of drag we can tap into airframe, flaps. gear, speed brakes props (esp VP) so we are also tapping into that lot.
Flaps give us lift as well as drag enabling us to land at slower speed.
Going down hill where we are using the potential energy in the airframe means that normally we will use drag to help us slow to a VREF speed by adding chunks of drag on our way down.
going down hill why would we want to remove drag unless we are wanting to go uphill again?
If we loose the engine we now have only one source of energy on a still day from the airframe!
We no longer have an energy source from the engine so trying to fly as far as possible while using as little energy from the airframe means we need to be as clean as possible.
Add drag and we have to use the airframe throttle i.e. the control column by pitching down to get enough energy to combat that drag. Pitching down gives us the energy but looses altitude.
Engine out we should only add drag when we are assured of a landing and sure of the glide path that drag has given.
Get that glide path wrong and the only way to extend the glide to reach a landing spot is to remove the drag or pick another landing spot on our descent profile.
But that is extreme! Normally going downhill add drag in a stabilised approach don't remove it ? Sorry if this is simplistic its not directed at our more advanced posters here
In the Citation if I want a steep descent rate or need to control speed pop the speed brakes (another form of useable drag) If my thrust levers are at idle or I need some thrust for engine anti ice . Levelling I will pop them away again think energy management
I suppose the ultimate drag is in the cirrus which gently lowers the aircraft to the ground under the caps
Pace