Ralph, I will float my thoughts based on what you have observed.
The spring is a shallow curve with hooks on the top end. These hooks engage a T in the action that applies tension to the trigger. With the screw in the full in position the arch of the spring causes the two ends to be closer together. Picture a piece of computer paper laying flat on a table and measuring 11" long. Lift that piece of paper in the middle (representing the strain screw) and the two ends get closer together. As the screw is loosened, the main spring gets longer and applies more lift or tension on the hook end thus raising the trigger pull weight.
The bottom end of the main spring is fixed at the bottom of the frame and cannot move so all movement is directed at the hook end. At least that's how my brain sees it.
