The term sequential is generally understood as a transition in which the legs each land and takeoff in a particular order, or sequence, though it can also be used to describe only part of a transition in which a landing or takeoff occurs in sequence.  Sequential transitions are extremely common within vert kicking, as well as to segue from an invert axis to a vert axis, and are becoming more and more popular in the inverted realm.  These transitions can be described as similar to walking and running, or shuffling of the feet, and in many cases feel just as natural and potentially powerful.