If #1 is rocking, both valves are slightly open. That's the beginning of the intake stroke. If the plug fires then, whatever fuel is in the cylinder fires, but the pressure escapes out the intake manifold, the exhaust manifold, or both. The plug is supposed to fire one complete turn later, after the cylinder fills with mixture, and it gets compressed by the piston moving to the top with both valves closed. Now, the burning fuel can push the piston down to the bottom, and the power it produces goes through the clutch, the transmission, and on to the wheels. The burned gas gets pushed out as the piston comes back to the top with the exhaust valve open. It takes two turns of the crankshaft (4 strokes) to get the whole sequence completed.
The #6 cylinder is doing the same thing as #1, but it's one full turn out of phase. When #1 is on top with both valves closed, ready to fire the spark plug, #6 is coming to the top of its stroke on exhaust. The exhaust valve is closing, and the intake is beginning to open. By watching the #6 valves, I know that both pistons are on top, and I can time the distributor to fire #1. It's a quick, simple way to estimate where the pistons are, and which cylinder needs the spark. You also don't have to mess with that hard-to-see ball on the flywheel, which can't tell you which cylinder needs to fire, anyhow.
Jerry
