The second form of the missleg, this transition is better described as a one legged punch.  It is an instant rebound from landing on one leg into the takeoff of the next trick.  Rapids are fairly uncommon, but have been growing in popularity since the advent of the feinted cheat kick, and their use by high profile NASKA competitors.  One appeal is the inherent change in rhythm that occurs with such a fast transition that typically has very little height or rotation following it.  Rapids are often used in multiple kicking combinations that stay more or less on the ground, such as narabongs and autobahns.  Some would argue that the skip transition is actually a succession of rapids, or at least utilizes a rapid in a compound transition, and that understanding the mechanics of the rapid will increase the potential of the skip as well.