Your friend mis-read, or misunderstood, about reversal. It's not that there is a control reversal, but the back side of the power curve is referred to as the 'region of reversed command', not 'reversed control(s)'. This is what I think you & your friend have misunderstood
This refers to the inability of the aircraft to return to a previous state if disturbed from that state ie is unstable in one parameter or other. Others have described the speed instability that occurs in this area.
Normally ie on the 'front' side of the power curve, an aircraft is speed stable. This happens because on the front side of the curve total drag increases as speed increases, and reduces as speed reduces. If the aircraft experiences as speed increase then drag increases. The increasing drag 'self corrects' the speed increase until the speed is back to original. If speed reduces then drag reduces. Less drag allows the aircraft to accelerate until it is once again at the original speed. The pilot doesn't have to do anything to correct a speed change thanks to it being self correcting.
In the region of reversed command this doesn't happen. Drag increases as speed reduces, and decreases as speed is increased (until on the front side of the curve again). If the aircraft experiences a speed reduction the increased drag results in increases drag and a continuing speed reduction. The reverse for a speed increase. Now the pilot has to actively make control inputs (power &/or pitch) to keep the speed constant.
Last edited by Tinstaafl; 31st Mar 2013 at 16:29.