To add my understanding, and perhaps more confusion. With No1 at TDC and the rotor arm pointing away from No1 contact in the distributor cap, the exhaust valve is about to close and the inlet is staring to open. With normal valve clearances, both valves will be very slightly open so no gaps between rocker and valve stem. By opening the clearance up to 1 mm for both valves, although the rockers will have started to move, there will still be a measurable gap, which if the cam is timed correctly, will be exactly the same for both the inlet and exhaust valves. If the gap to the inlet valve is smaller than the gap on the exhaust valve, the timing is advanced.