And you don't have to tap into potential energy you just need to reduce your AofA which will reduce your rate of climb but it won't be required to decend. In fact recently this exercise was added into the UK syllabus as one of the set exercises.
No you dont HAVE to tap into potential energy but effectively consider this?
Forget jets you are flying a small aerobatic machine you have full power but your climb is so steep that your speed falls off sharply.
Now the speed had dropped way below stall the nose falls! where are you getting energy from to get enough air moving over the wings? Just the engine?
I always think of an aircraft having two power sources and two throttles. One the conventional throttle the other the elevator which allows you to tap into the potential energy in the airframe.
How does a glider in still air with no lift and no engine break a stall?
Pace