I'm glad you like it Burr!

It works by "Knot insertion" which basically means the surface will retain its same shape from before the point insertion but the control points will shift around with a new row and/or column added in the general area of where your target point is at.

If you use it on a very simple surface like a plane the first pick will end up adding a bunch of points because it also does what's called "degree elevation" to switch from a degree 1 surface (degree 1 is like a polyline, straight lines only between points) to a degree 3 stretchy surface, and that degree elevation step also adds points in too.

- Michael