The thumb screw allows you to tighten or loosen it without a screwdriver.
You can rotate the handle clockwise to tighten the screw and rotate the handle counterclockwise to loosen it.
If you can't tighten or loosen the screw because the handle is blocked, you can do as follows.
1. Hold the handle to draw it away from the thread part.
2. Rotate the handle to stop it from being blocked.