Click the icon and then click on any control point in one of the orthogonal projection views. A new row of control points will be inserted with coordinates coincident with control points of the row that the user clicked.

For Bezier surfaces, duplicate control points increase the weighting in that area of the surface, and the resulting interpolated surface follows the control points of the duplicated row more closely.

For Catmull-Rom surfaces, the resulting interpolated surface folds back and passes through the control points of the duplicated row a second time. This may result in the surface self-intersecting.

Switch between orthogonal projection views and continue duplicating rows.