Click the icon and then click on any control point in one of the orthogonal views.

The surface will be split into two at the row corresponding to the control point that was clicked. Note that the split row is duplicated, so that each of the two surfaces resulting from this operation contain that row of control points.

Note that this operation is only permitted if the two resulting surfaces comprise the minimum number of rows and columns for the surface type (2 rows and 2 columns for a Bezier surface, and 4 rows and 4 columns for a Catmull-Rom surface).