This extension works on the simple theory of Fitts' Law. For all of you who are aware of this law, you would know that the time to acquire a target is a function of the distance to and size of the target, the target here being the two *small* back and forward buttons on top left of your browser. This implies that acquiring these targets, ie, pointing to these buttons is not very easy. All the more difficult if you are using the smaller buttons so as to get more space for web page content.
But withthis extension our 'target' effectively has considerable height and an infinite width! And thus one need just move the cursor with high velocity to the left and navigation is much easier.
Usability Glossary: Fitts' Law