There are a few words in that description which might seem a bit technical to a lay-person.
The word "pitch" refers to the angle of the aircraft, either up or down.
pitch: (of a moving ship, aircraft, or vehicle) rock or oscillate around a lateral axis, so that the front moves up and down.
"49°" refers to the angle from the horizontal (0°). 49° is a very steep decent. Much more so than a normal descent profile.
The "elevator" is the control surface used on an aircraft to control the pitch, either up or down. So "nose-down elevator deflection" says that the elevator position were such that the aircraft is pointing down. The implication/possibility is that the controls in the cockpit were commanding that input, or that a mechanical fault caused the elevators to do so.