You can watch a live action demo from one of my students in the video clip to the right.  Although he moves across a room, this can be done backwards and forwards on the spot just as easily.  You can also make the excerise harder or easier by changing the surface you work on.

The sidewinding action simulates the movement of a snake and allows us to develop a natural movement on the ground without the direct use of our hands and legs for leverage or traction. So why is this useful?  Two main reasons: 1) Especially whilst executing BJJ / ground-based techniques, our arms and legs may well be busy; and 2) Throughout all the different aspects of martial arts, shoulder and hip rotation / twist is a vital movement.  Sidewinding is also excellent for building core strength.