After staring the applet, click with the mouse into the field to add a point to the polygon. Another feature is to drag and drop the points (you have to pick the little squares), to change the curve.
Hitting the Init button means to remove all points. Using the Show points option, you're able to show or hide the little squares. If you like to see the spline a bit different you may choose the kind of polygon you'd like to see:
All source codes as well as executable programs provided by this demo are free to use, copy, modify and redistribute in any way you like, but on your own risk.