Electricity will always want to go to ground through the path of least resistance. For ignition systems, this means that energy coming out of the secondary will want to create a spark outside the combustion chamber if a low resistance path is present. This video is sponsored by Blue Streak.
When the power from the secondary is traveling through the boot, ignition wire or spark plug, it creates static electricity that causes oil and dust in the area to stick to the component. You’ve probably seen this condition on spark plugs where the ceramic insulator is exposed on the side of a head. The spark plug can have a yellow or orange band that is called a corona stain. Some technicians mistake it for exhaust gases leaking between the shell and insulator. If enough material builds up on a boot or plug it can create a path of least resistance for the electricity generated by the secondary. This is also the same phenomenon that causes carbon tracking inside a distributor cap.
If the ignition coil is mounted in the head above the spark plug, the possibility of carbon tracking is reduced, but not entirely eliminated. If the valve cover gasket or spark plug tube seal is leaking, the oil and vapors could cause carbon tracking and other damage eventually. If the boot does not fully seal against the top of the tube and insulator of the spark plug, oil and dirt can find its way on to the plug and boot. This is why a very light coating of di-electric grease is recommended. However, too much di-electric grease can attract dirt and oil without the help of static electricity.