As well as being able to watch the simulation in progress you can also modify it. If you select a point by left clicking you can drag the point and move or stress the structure. If you right click on a point it vanishes and often gravity takes over. If you hold the control key down then the simulation pauses while you edit it. You can add points by left clicking and Alt and left click adds fixed points. It's a lot of fun as you can see in the video but its even more fun if you actually try it out.
The Physics engine is called jsVerlet and it implements Verlet integration to solve Newton's equations of motion. This particular method is more stable and easier to incorporate constraints than alternative methods. As a work in progress it is impressive, but for added fun some collision dynamics would be a step in the right direction.
Another Verlet Physics engine that you might consider building into your own programs is verlet-js. This allows distance and angular constraints which, from the examples provided, are enough to model a great many things. There are four examples provided - basic falling bouncing shapes, a fractal tree which blows in the wind, another cloth but not tearable this time, and a spider's web complete with spider that will upset any arachnophobes out there.