Where is the 'zone of reversed commands...'?
This is commonly known as the Speed Stability, not to be confused with speed stability in the sense of static longitudinal stability. I think there's some common confusion with this phenomenon. The first part of this answer addresses the OP's question; the second part clarifies a common confusion.

1. Main Answer

In level flight (i.e. zero vertical rate), the longitudinal equation of motion can be succinctly written as:

$$m\dot{V}=T-D$$

$m$ is airplane mass, $V$ is airspeed, $T$ is thrust and $D$ is drag. Now if we express thrust and drag as first order approximation as a function of change in airspeed ($\Delta V$) from the trimmed condition, we have:

$$T=T_0+\frac{dT}{dV}\Delta V=T_0+T_V\Delta V$$ and $$D=D_0+\frac{dD}{dV}\Delta V=D_0+D_V\Delta V$$

At trim condition, we necessarily have $T_0=D_0$. So now we have a new equation of motion:

$$m\dot{\Delta V}=(T_V-D_V)\Delta V$$

This equation is a first order ordinary differential equation, and is stable if $T_V-D_V&lt;0$ and unstable otherwise. 


For a jet plane, thrust is fairly constant in flat rated conditions and $T_V$ is approximately zero. Thus, $D_V=0$ corresponds exactly to minimum drag or minimum thrust required (where $C_{D_0}=C_{D_i}$ for high aspect ratio, low Mach airplanes).
For a propeller plane, power is constant, but now the stability criterion is $-\frac{P}{V^2}-D_V&lt;0$. This corresponds neither to minimum power required nor minimum thrust required.


2. Addendum

What does this result mean, exactly? If an airplane is trimmed in the speed unstable regime, will it decay toward stall if it experiences a speed perturbation with pilot hands-off, even if it's statically longitudinally stable?

Remember, the equation we began with only holds in level condition where the airplane is neither climbing nor descending. Thus, the pilot must be holding altitude with elevator while the speed is changing. The conclusion for speed instability is:


If the altitude deviates below the trim altitude, pulling up will decrease the airspeed, resulting in further energy deficiency. Speed will continuously decay in this manner.
If the altitude deviates above the trim altitude, pushing down will increase the airspeed, resulting in further energy excess. Speed will continuously accelerate until reaching a stable point on the other side of the polar.


Since the pilot is in the loop, it has nothing to do with the basic aircraft eigenmodes.

The same result can be obtained if we aim for constant flight path angle. In approach configuration, this is also called flight path stability, which I think is a better name than speed stability.