As a magneto rotates, the interaction between the lines of magnetic flux and the wire conductors in the coil is continually reversing. As a result, the voltages generated by the coil are alternatively positive and negative voltages. On twin cylinder magnetos, both these voltages are utilised thereby delivering a positive voltage to one spark plug, and a negative voltage to the other. This means that the spark jumps from the centre to the side electrode on one spark plug and in the opposite direction, from the side to the centre electrode, on the other. The design of a spark plug is such that it’s centre electrode (surrounded as it is by a ceramic thermal insulator) is always hotter than the side electrode (screwed directly into the cylinder head which acts as a heat sink). Now, the laws of physics show that electrons are more easily emitted from a hot electrode to a colder one so a spark jumping from the centre electrode to the side - caused by the negative voltage - is always stronger than the other. This picture of the sparks from a six cylinder magneto clearly shows the difference between the alternate sparks: