I was completing the compression test on #4 cylinder on the 0-200, I had the cylinder at TDC with 80 PSI on the tester gauge and it was showing 74/80. I ws standing on the left side of the engine the owner was in front of the prop, I held the tester up so the owner could read the numbers, he leaned in to better see the gauge and pushed the blade down,bring the piston off TDC and WACK. it happened so fast I didn't have time to do any thing by ask " Are you all right, did it hit you" he responded Yes, while taking off his hat, blood was already running down his neck.