To complete the aircraft - automobile analogy, consider the auto on a hilly road.
You're driving at first on level ground, then start up a hill. In physical terms, you are putting potential energy into the vehicle; but if you are producing constant torque, some of that torque now goes into lifting the vehicle, leaving less available to propel the auto. So the auto slows involuntarily.
Rounding over the crest of the hill, now that stored potential energy (a redundant ezpression,
) is helping propel the auto, so it speeds up even faster than when on level ground.
The same thing (almost) happens in aircraft.
In an auto, the cruise control will adjust throttle position to vary torque to maintain approximately constant rpm and vehicle speed. In an aircraft the CSU varies the load on the engine by adjusting prop blade angle, and if the throttle is not moved, torque will remain constant as a by-product. (Analogous to a variable-ratio gearbox in a car).