The ignition coil is the part of the engine that generates high voltage to power the cylinders. Distributor ignition coils transmit high pressure from the coil to the correct cylinder. The connection is done through a cover and rotor. The ignition coil is attached to the rotor, which rotates inside the cover. Each time it passes a contact in the cylinder, the coil sends a high-voltage pulse through the rotor, sending its power to the engine. The pulse travels through a small gap between the rotor and the contacts and travels down the spark plug wire to the spark plug on the cylinder.
Some older distributors have an extra section on their bottom. This part destroys the current flowing to the coil. The ground side of the coil is connected to the breaker point, which is connected by a lever to the cam in the middle of the distributor ignition coils. When the lever is pushed by the cam, the coil loses ground and generates a high voltage pulse. The breaker point is also responsible for the timing of the spark.
Timing is critical for proper engine performance. There is usually a short delay between when the spark goes out and when the fuel and air mixture in the cylinder reaches its maximum pressure. Spark must happen before the piston reaches the top of its compression stroke so that there is enough pressure in the cylinder for the spark to be effective. If the spark occurs when the piston is at the top of its stroke, by the time it reaches the mixture, the piston is already on its way back and maximum pressure will be lost.