The Electric Trim control is a "split" switch located on the control wheel. This allows the plane to be trimmed by your thumb, not using the trim wheel, located on the pedestal. Moving the switch up or down activates pitch trim servo which also results in movement of the trim wheel.
When the autopilot is engaged it controls the pitch trim servo, when is not engaged, electric trim may be freely used to command the pitch trim servo motor instead of autoplilot. If trim switch is moved while autopilot was engaged, it automatically disengages the AP (roll and pitch), not affecting the FD.
You always should move two switches to control the trim servo, moving just one side of the switch shouldn't actuate the trim. If you hold one side of the split switch down for more than several seconds (depending on aircraft), the trim switch will automatically become disabled.
Trim switch are split by two parts for safety. If it was a single switch you could accidentally bump the trim switch with your thumb at the very wrong time (and disengage AP). Also, a switch could getting stuck and lead to fast dangerous overtrim.
The left switch is the "ARM" contact, it can be a simple safety contact or clutch control switch for trim servo, the right switch controls the UP and DN contacts to move servo.
Also, electric trim movement speed can depend on current airspeed, so the trim movement slows down when airspeed increases for more consistent response.
For electric trim there a special SImVim function, that allows you to make ET in your cockpit the same way as in real aircraft. That's mean that with two-way trim switch you control the DC motor that rotates the pitch trim wheel.
Note: the same DC motor is used for autopilot pich trim commands to rotate the pitch trim wheel. In this case the encoder connected to the wheel is deactivated while AP is engaged.
A simple and obvious trim switch implementation is using the right switch as up/down trim commands actuator, and the left switch - as a serial interrupter for the "command" switch circuit:
As you can see, to make these two-way switches you can use either a pair of momentary buttons (left circuit on the picture) or spring-loaded 3-position rocker switch (right circuit).
To assign the electric trim function in the configurator find the "Electric Trim UP / Dn" parameter (Elec_Trim) for 3-position (or two button) switch.
The "Electric Trim ARM" parameter is not used for this moment, but can be implemented later (to add an output signal for the DC motor clutch activation) if there will be demand from users.