The diagram is from NASA TM X- 73,229 which is about recovering hang gliders from dives, but has broader implications as well as a fair bit of aviation history behind it.
Do you see a connection with the TAROM Airbus over Paris, the Interflug A310 on approach to Moscow, and for that matter what the AF447 crew would have faced in recovering their aircraft to level flight if the trim did not run down quickly enough?
Just trading potential and kinetic energy.