Turning the screw in either direction doe not open or close the gripper. If you screen it in the end of the screw hits the plates sooner and limits the travel so the gripper will not close as far. If you screw it out the gripper travels farther. The farther is travels when tension is pulled the more it closes down on the string.
I have the ability to drive people crazy, not sure if I was born with it or I learned it but I'm damn good at it.