Naihanchi kata [内蹯地型 or ナイハンチ型] has apparently simple footwork moving side to side using a "horse-stance" (naihanchi-dachi) [内蹯地立] and a "cross-stance" (kosa-dachi) [交差立]. However the horse-stance and cross-stance are used in application by rotating the body as one assumes these stances. In Shorin ryu these evasive body rotations are often referred to as "body-changes." "Body-change" is the English translation of Tenshin [転身] meaning "body-rotation" (also sometimes called tai-sabaki [体捌き]). The primary purpose of a "body change" is to move out of the way of an initial attack and to make a renewed attack more difficult. However, an often ignored aspect of these "body-changes" is that they add considerable power to techniques. This is especially important for tuidi techniques.
The reason that the kata doesn't illustrate all the possible footwork explicitly is that most of the techniques can be applied with more than one type of footwork. In fact, there are 20 basic pivoting methods that I use with Naihanchi (and innumerable variations as well as more advanced forms of footwork). Rather than illustrating all the possibilities, the kata indicates these in a somewhat abstract way. Below the basic kata footwork is shown in a short clip and is illustrated by a diagram below the clip.
The diagram below shows the two most basic ways to use the Naihanchi "cross stance". In the first example you leave the left foot unmoved and pivot on the ball of the right foot as the hips and shoulders pivot clockwise. In the second example the left foot still remains unmoved but the right foot steps behind as you pivot clockwise.
In the diagram below the footwork is similar to that illustrated above, but in each example the left foot takes a step before the body pivots in a clockwise direction as the right foot moves into the "cross stance"
Here is a video systematically presenting the 8 kosa-dachi stepping patterns shown above:
Below are the two most basic stepping patterns for pivoting the body into a "Horse-Stance":
All 10 of these stepping patterns can of course be used by pivoting anti-clockwise as well (making 20 basic stepping patterns).
Some (but not all) of these stepping methods can be seen in this video: