Pilot performance was as bad as it gets, and call’s Ethiopia’s training into question. They obviously don’t teach their pilots anything about the trim
With the advent of the Boeing 737 Classics, Boeing withdrew its advice published in the original Boeing 737-200 Pilot Training Manual (PTM) on use of the manual stabilizer trim wheel to counter a runaway electrical stab trim motor. This was the so called "Roller-Coaster" or "Yo- Yo method of recovering from an extreme stabiliser position. Previously this method was taught in the Boeing 707 for the same reason. In fact it was even used for airborne demonstrations with certain limitations.