I would explain it like that.
optimum cruising altitude: is where we can set optimum thrust (90-95% rpm).
there we will have the optimum SFC. AND at optimum cruising altitude the thrust will be equal to maintain the optimum angle of attack.
OK, so now imagine you are cruising at optimum cruising altitude. Hours later you will have burned quite a lot of fuel, and thus the aircraft is lighter. So what will happen?
The angle of attack requiered to maintain altitude will be less due to the less weight. So if you maintain the thrust setting the aircarft will speed up.
in normal operation you would now climb to a higher cruising altitude where the thrust produced again gives you the optimum angle of attack.
Now one can clearly see if I fly below optimum cruising altitude, I will fly faster but not economic.