Hey, you got pretty far if you made it all the way to the \sum!

To put it more simply, think of the trust graph as a series of pipes between each user. Trust is a liquid that flows across these pipes. Some pipes have stronger flow than others, based on the similarity of the two users' zapping behavior. If you let the trust keep flowing across these pipes, it eventually settles on a stable amount of liquid in each person's tank. That's pretty much how it works.