As I am coming to understand it, or think i am, there are two ways of cycling:
Normal--moving from one mood to another through a normal period
Straight--moving straight from one mood to another without passing through a normal period

The other thing I "think I know" is that rapid cycling means changing more than four times a year. My mind expects that if there's a "rapid" then there's a "medium" & "slow" but apparently not...

RR