From FCOM Vol 2. Systems Description
Primary Pitch Trim Control
Primary pitch trim is controlled by the dual pitch trim switches on each control
wheel. Both switches must be moved to command trim changes. The primary
pitch trim switches are inhibited when the autopilot is engaged. Pitch trim does not
move the control column.
In the normal mode, primary pitch trim operates differently on the ground than it
does in flight. On the ground, the stabilizer is directly positioned when the pilot
uses the pitch trim switches. In flight, the pitch trim switches do not position the
stabilizer directly, but make inputs to the PFCs to change the trim reference speed.
The trim reference speed is the speed at which the airplane would eventually
stabilize if there were no control column inputs. Once the control column forces
are trimmed to zero, the airplane maintains a constant speed with no column
inputs. Thrust changes result in a relatively constant indicated airspeed climb or
descent, with no trim inputs needed unless airspeed changes.
When pilot trim inputs are made, the PFCs automatically move the elevators to
achieve the trim change, then move the stabilizer to streamline the elevator.
Stabilizer motion may also automatically occur to streamline the stabilizer and
elevator for thrust and configuration changes.