Primary and Secondary Pitch=Control
In case anyone is getting the wrong impression, let me chip-in an important point to this THS discussion.
BOAC is focussed on discussing the THS as if it is used as a primary flight control. This is not the case on FBW Airbuses, just as it is not the case on an A300 or A310, or on B707 through B777. They are not like Gums's Viper, which uses the whole horizontal stabiliser (tailplane) as an elevator.
On all these jet transports, primary pitch control is provided by the elevator, as per DHC-1 Chipmunk or Cessna 150. On those aircraft, the pilot then adjusts a trim tab on the elevator itself, which provides an aerodynamic load to hold the elevator at the position the pilot wants. This position is reflected by the control stick.
On large jet transports, speeds, CG and high-lift devices are so much more variable that, on the approach, the elevator would be in a very high position. In other words, the primary control of pitch would be already near the limit of its travel. This is plainly unsatisfactory. The solution is to provide a secondary pitch control, in the form of the all-moving horizontal stabiliser, which trims the aircraft in pitch to enable the elevators to return to neutral. The control column will also return to neutral. On Airbuses, FBW or not, this is referred to as the THS.
When hand-flying an A310 or B767, the PF makes pitch commands with the control column. The elevators move. When the PF achieves the pitch effect he/she wants, he trims the THS to enable himself to relax pressure on the column. All jet transports have an autopilot (AP) which uses the same technique as the pilot. When it trims the THS, we call that auto-trim.
On a FBW Airbus, the AP works much the same, using auto-trim. When hand-flying, the PF does not need to trim the THS manually: the auto-trim does it. (Several American jets had a crude version of this 40 years ago, called control-wheel steering [CWS]). This only applies in Pitch-Normal and Pitch-Alternate laws. In Pitch-Direct law, the PF does his own THS-trimming.
AF447 left its cruise altitude in Pitch-Alternate law, and most of us think that this was retained for the rest of the flight (but lacking any high-AoA protections). The EFCS interpreted the PF's stick commands in pitch according to its C* terms of reference, applied the appropriate elevator when necessary, and then trimmed the THS to enable the elevators to return to neutral. The reason it trimmed the THS all the way to minus 13 degrees (13NU), was to avoid reaching full-travel UP on the elevators.
At any stage during that up-trimming process, or even after it, what would have happened if the PF had pushed the stick fully forward? The EFCS would have selected full down-elevator, and started trimming the THS in the nose-down-trim direction.