In FM there is a reversal (signal level, or magnetic flux) at the start of every data bit time (i.e. every clock time); for a '1', there is an additional reversal in the middle of the bit time, with no reversal indicating a '0'.
Consecutive zeros show a reversal between data bit times; consecutive ones show two reversals in every bit time (i.e. doubling the frequency of reversals; hence the name).
It was mostly replaced by Modified Frequency Modulation, which can store twice as many bits in the same maximum reversal rate.