This script allows you draw a control-point curve on a surface, where you can dynamically change the amount of points and see a preview of the resulting curve while drawing. Normally when you draw a curve on a surface, Rhino will make an interpolated curve on the surface, where the amount of points is dependent on the absolute tolerance. While this results in a curve that is actually on the surface (within set tolerance) it also means the curve can become quite complex/dense.