Perhaps the most common method to rapid currently is the feint kick followed by a rapid round kick. A feint is a kick where the kick is not actually executed, but instead is faked as if it was going to be, then canceled. Upon landing on the inside leg, with the outside kick chambered, but not extended, an instant punch occurs and the legs switch into a round kick. Typically, using mainstream terminology, something like a cheat-720-feint is followed by a rapid round kick. Some people have taken to calling this combination a rapid-9, or rapid-900, however this is incorrect, because a rapid is a transition, and does not create a continuation of the prior move. A cork swingthrough cork is not called a double cork, thus using cumulative numbering of vert kicks would be similarly inappropriate.
Notice that the rapid and missleg in this above clip are incredibly similar. This is more than an execution issue; this is because they are basically the same transition. Rapids should be faster and more direct than misslegs, which tend toward a degree of absorption in the landing, with the in-air leg sort of bouncing into the next trick.