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 would like to apologize to anyone I have not offended. Please be patient I will get to you shortly.