Mostly back in the day of manual chokes. Pulling the choke on a warm engine and firing the plugs in the exhaust would make a flaming effect as the raw fuel was burned. These plugs were fired using a separate coil set-up.
I wouldn't try it on a modern engine. Plugs (the ones in the engine) foul too easily and don't readily self-clean. Unlike point/condenser or magneto ignitions.
