This is the knob for the ball and joint on PowerStick-53" Gen1 or Gen2.
Note: PowerStick-53" Gen1 (aluminum ball) was NOT designed to become fully tight. It was designed so customers to make quick adjustments to the GoPro or other action camera being used. Standard operation: Turn the knob until you feel it getting tight and only turn 1/4 turn more. Anything past this point will actually make the ball and joint more loose. The tighter you turn this knob, the less tight the ball and joint becomes.
PowerStick-53" Gen2 was designed to get more tight, but still leave the ability to quickly turn the camera so you don't miss the shot. Turn knob to desired tightness.