The FDR showed that the captain immediately applied back pressure on his sidestick to arrest the pitch-down movement. The aircraft’s flight control system did not initially respond to the captain’s sidestick input, but after about 2 seconds the aircraft responded normally and the captain commenced recovery to the assigned altitude. During this 2-second period the aircraft descended about 150 ft. Overall, the aircraft descended 690 ft over 23 seconds before returning to FL370.