[Raceghost]

To do this, that is correct, ducktape a spark plug to the exhaust, this will ground the outer part of the spark plug to the end of the exhaust pipe, then put a hot wire to a switch in the car and wire it to the battery. switch so you can turn it off when chased by cops or rice haterz for having a marshmello roast at the local tire store.
The reason this works, is your engine does not burn all gas, the excess is supposed to be chemically changed when it hits your cat. so it works better on higher powered engines that push the gas or exhaust gasses right pass the cat and into the open air. The hot on the spark plug will ignite the gasses, causing a flame burst that looks cool to some people.

The real way to succeed, is to just cut the exhaust pipe off before the cat, and push it right out under your door, then there is no cat, and the engine realeases flame bolts because of no back pressure.

Have fun, singe your mechanics hair when he looks under there.