Here's a picture gently stolen from Apple showing exactly what this extension enables.
This page is a place where you can experiment with the extension. A two finger gesture towards any direction will reveal an arrow which activates a function when it slides all the way across the page.
Eventually this might let you add your own custom actions (for instance, launching bookmarklets). If you really want this, please email me at antimatter15@gmail.com or file an issue on Github.
Here's a picture gently stolen from Apple showing exactly what this extension enables.