Checked you tube and PPRuNe... Well there you have it.
Why don't you give him the answer then you muppet!
Unbelievable, why even bother to reply with such a useless answer to a set of perfectly valid questions?
Statement: I understood that on 777, pushing the trim switch located on the yoke does not make the stab moving directly -- as it does on 737 --. Instead, it changes the trim reference speed, the speed which is targeted by the current configuration by the trim. For instance : I am at FL080, and I want to descent and keeping the speed, so I just reduce the throttle and push forward. Now I want to make the same but I want to reduce the speed then I push forward and trim downwards -- because I want the reference speed to be lower!-- at the same time (seems contradictory, isn't it ? On a C172 I would trim upwards to get the nose down...), so in this situation I don't know what to do...
From the B777 FCOM
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.
By pressing the trim switches you have the ability to increase/decrease the reference speed at which,
"the airplane would eventually stabilize if there were no control column inputs". You are properly trimmed on the B777 when the aircraft is maintaining a constant speed without any control column inputs.
Example, you are flying at 250kts and you are having to push the control column slightly forward to fly level. In this case your trim reference speed is "too slow" hence a few clicks of down trim, will increase the trim reference speed until no further downward control column input is needed to maintain level flight at 250kts.
Set the desired pitch, then increase or decrease trim reference speed to the point where it matches the speed you are trying to fly at with a neutral column input.