The Electric Trim control is a "split" switch located on the control wheel. This allows the plane to be trimmed with 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 electromagnetic clutch control switch for trim motor, the right switch controls the UP and DN contacts to control the DC motor direction.