Pulling the trigger rotates the trimmer on the servo tester. The gears ratio has been designed so that pulling the trigger (a movement of about 25mm) is converted into 270 degrees of rotation of the trimmer.
To get it work on first try, I measured the PCB and size of all the components of the servo tester. Then I created a 3d model in Solidworks of the servo tester. I downloaded the Step files of some gears with the correct ratio from SDP/SI, then modified them to fit.