Khan Academy interactive continuous rotation servo demo
Khan Academy user mbbackus created a demo that allows users to visualize the effects of sending different pulse widths to the SM-S4303R and Parallax continuous rotation servos. (You can use the up and down arrow keys to change the pulse width, and more instructions can be found as comments in the code.)
For those who are interested, you can learn more about standard RC servo pulses in this series of blog posts about hobby servos.
You can see the demo on its Khan Academy page.