Before we continue on, it is probably good to get out of the way, what the thread trimmer is. In the picture (without text overlay) you can see a small elliptical like washer with a hex bolt in its center, and an arm below it, next to the feed dog, this is the thread trimmer (probably like a top “cap” [?]). Essentially the thread trimmer is a small cluster of bushings which are driven by a simple mechanism to control a small knife. When the thread trimmer is activated it tells the knife to sweep across the thread - underneath the throat plate (or needle plate, what the material is running over, which has the grooves cut for the feed dog to pass by) - and trim the bobbin thread. How it trims the upper thread coming in from above is unclear. What a thread trimmer is good for is that; it saves a lot of time from having to constantly pull the fabric out and trim each side separateley (after your seem is made), it cuts very close to the fabric (probably closer than a human can), and it leaves the remaining thread at the optimal length to begin sewing again (this saves thread, and saves the person sewing from having to always pull a little excess out before laying down the next seem).