It gets murlier and murkier the further that we go in this discussion.
I thought I was satisfied several pages back and posted my undrestanding of the issue for any further enlightment of myself. Since then I have seen a lot of personal semantic arguments between a few individuals. So I would propose adding some new paricipants eager to learn communication skills which might satisfy most of us lurkers.
I'm really not sure that I yet have this correct but here's where I'm at right now.
There are two parts of the engine as a propulsion system assisting in the braking of the aircraft.
The first is the deflection of some of the engine's propulsive force (energy) forward against the motion of the aircraft. This is accomplished by turning the air entring the inlet by more than 90 degrees (reverser cascades, buckets, clamshell deflectors etc.) This is only practical at high aircraft speeds due to the inherent risk of reingesting the air and runway debris.
The second means of assisting the braking of the aircraft is to increase the engine nacelle drag (frontal area of the engine) by slowing the air down that passes through the engine without producing significant forward thrust or turning the reverser air forward enough to cause reingestion.
This is accomplished by an idle condition on a running engine where the compressor and fan work are just enough to slow the inlet air down and maintain the engine cycle with very little net thrust produced. Thus the effective drag on the engine nacelles assume a plate blockage (not just the lip) while at the same time without effective rearward thrust the net effect is almost entirely a perceptive increase in drag assisting the brakes.
If the reduction of the engine reverser efflux is done at low aircraft speed and low thrust settings, the pilot should perceive no changes in retarding force.
OK, I'm quite sure that I haven't got this all correct so anybody is welcome to tweak it or entirely discard my understanding. Either way I should learn something more from the discussion.