This is a guide to get the piston ring square in the cylinder bore so you can measure for ring gap. Put the ring in the cylinder, and push it down with this tool.  Then reach through the tool and pull the ring against the bottom of the tool to ensure its seated properly. This tool will make sure you're below the piston ledge in the bore, and that your ring is square with the cylinder walls. Now you can remove the tool and use feeler gauges to measure ring gap.