Each time a cylinder fires it must initate the fire from a high voltage supply in order to burn as much fuel in the cylinder as possible. Once the process begins the coil pack builds up energy (as much as 50,000 -75,000 volts), and when the igniton control module gets the signal from either a distributor or crank sensor to release this engery, it is released through high tension (plug wires) cables and travels to the spark plug. The spark plug has a gap between the voltage and the ground. Since this voltage travels to ground, it jumps the gap of the spark plug to ground causing a hot enough fire to explode the fuel/air mixture inside the cylinder chamber. If the coil, or coil pack/packs are bad there will be a loss of fire to one or more cylinders (depending on the system your vehicle has incorporated) causing a failure to fire the cylinder/cylinders causing what is known as a miss. This miss actually creates a drag on the engine rather than helping the crankshaft turn.