Note that the download differs from the demo version in that it records the username of the voter, whereas the demo records the IP address - for obvious reasons.
If you look at the code you'll see that it records a list of who has voted in a field on the document itself. This was an educated choice I made based on circumstances. The client I was working for have between two and three hundred employees. In the unlikely event that all of them voted on one document it could still cope. If you don't know how many people might vote then you might want to store votes as separate documents — while keeping the overall score in the actual document — and change the agent to look-up to a view instead. Horses for courses, as always.