Small Engine Troubleshooting Problems? Ask a Mechanic Now.
A spark plug fires at the end of compression stroke and the power stroke. Piston goes down and draws in air/gas. Piston goes up compressing air/gas mixture and spark plug fires. Piston goes down. Piston comes up and releases exhaust and spark plug fires.
The keyway in the flywheel and the GAP of the coil is what controls the spark. If the keyway shears and the flywheel turns just slightly, the fire will be off and it will not fire when needed and engine will not start.
I am not sure I fully understand what you are asking.
What stops plug from firing at the top or exhaust/intake stroke? The flywheel key, Flywheel, and the coil.