Put simply, the ignition coil converts the low voltage of the motorcycle battery (approximately 12v) into the huge voltage required to jump the gap at the spark plug (around 20,000v). The coil is made up of a primary winding, which is a made from heavy gauge wire, and a secondary winding of thousands of turns of thinner wire. Current flowing through the primary winding creates a magnetic field, which oscillates when the ignition contact breaker opens, generating the larger voltage in the secondary winding. The huge amount of stored energy in the coil wants to find its way to earth and it does so by travelling down the bike’s HT lead and through the spark plug.
Many bikes have twinoutput coils to save space. These operate on a wasted spark system which means that both spark plugs attached to the coil fire at the same time but one of them sparks without fuel, so there is no detonation. All this happens at unimaginable speed – the coils on a sportsbike near the redline are sending 233 electrical pulses per second, or one every 14 milliseconds.