How It All Works
It's pretty simple: you get presented with a line roughly around at your rating, and if you scan it correctly, your rating goes up (meaning that next time you'll probably get a tougher line). If you scan it incorrectly, your rating goes down (and next time you'll probably get an easier line). There are more bells and whistles, but that's pretty much it.
Perhaps a more important question is: does it work? With my own classes, the answer is a simple yes. The key is to scan as often as possible, learn from your mistakes, and grow over time. To the right is the graph of one of my own student's progress as she learned how to scan. Other than a very basic intro and presentation of the rules of scanning (focusing on the two syllable rule), everything else is due entirely to using this website.
There's your rating, but there's also another metric that each user has called RD, which stands for "ratings deviation". Your rating tells us roughly how good you are at scanning, but there's some wiggle room. RD tells us how much wiggle room. As you scan more and more lines, we get more confident of your actual rating, and so your RD goes down. And since scanning is a skill, every day you don't scan, your RD goes up just a little bit. So if you take a month off, you'll very likely be rusty, and so your rating probably isn't very accurate.
RD is important, though, in figuring out your new rating. If you have a high RD, your rating will change a lot more than if you have a low RD, because with a lower RD (i.e., you have scanned a lot of lines recently) we are pretty confident of your true rating.
The key is, practice and practice often. That is the best way to increase your skill and rating.