The physlet is a java library designed to be controled with java-script.
So you can add java-script to add different effect you want.
You will need to know about basic javascript coding and you also need to read physlet document to know what javascript functions are available. (You can download/view document from physlet home site).

Let me know you some examples: (I also add the same code to the first message.)

Check out the above physlet applet after you click the button.

If you click [b]edit[/b] checkbox in the simulationBuilder page.
You will find out the javascript code being generated with that builder.
The builder only support basic functions in physlet library. Many complicated functions were not implemented with the builder (Because I create the builder for beginner to create simple simulation. And I do not have time to create complicate graphical user interface--- I am not a professional programmer.)