Need a bit of help understanding the BitTorrent protocol

So, I have a basic idea of how the BitTorrent protocol works. The thing that is confusing me is this :

"Data transfer takes place whenever one side is interested and the other side is not choking. Interest state must be kept up to date at all times - whenever a downloader doesn't have something they currently would ask a peer for in unchoked, they must express lack of interest, despite being choked. Implementing this properly is tricky, but makes it possible for downloaders to know which peers will start downloading immediately if unchoked."

This is taken from http://www.bittorrent.org/beps/bep_0003.html. It's under the peer protocol, I'm a bit confused about the direction of data transfer if one is interested and the other is choked. Also, I don't get the being interested part. Can someone explain this a bit more clearly?

Thank You :)

EDIT : This is not related to my school work in anyway. I'd really appreciate some help on this.