Program turns LPs into MP3s...via scanner?

I'm constantly amazed at the cool stuff geeks come up with:

This one may be a little nutty, but you have to admit nobody knew PCs could do it. In fact, I'm still not sure they can. This clever but crazy hack runs a virtual needle around a scanned image of an LP to create a .wav file of the recorded music. You won't want to rip your old LPs this way, though. Programmer Ofer Springer's demonstration file, a reconstructed recording of Vivaldi's "The Four Seasons," sounds like it's being played at your neighbor's house--across the street. Still, it's good enough that you can recognize bits of the melody. See this Web site for more, or download and compile the Digital Needle source code, fire up your scanner, and try it yourself.

How do it know?