I pulled this from google, but it sounds right going by memory. And I am getting old LOL! anyway you do this for a quick timing when your damper mark has slid and a timing light wont work. This is better than "timing by ear". I had seen some peoples timing by ear that was way off. LOL!
Get a vacuum tester and put it inline with manifold vacuum. Advance the distributor until you get the maximum amount of vacuum reading on the gauge. Then turn the distributor back exactly 1 degree on the vacuum gauge. Tighten everything down and adjust your idle if necessary.