If you do an operation on a group - such as reading an article, looking at the group table of contents or similar, then leafnode will go and fetch articles from that group when it next updates.
Since you have read this dummy article, leafnode will retrieve the newsgroup rec.woodworking when fetchnews is run the next time. If you'll look into this group a little later, you will see real articles.
If you see articles in groups you do not read, that is almost always because of cross-posting. These articles do not occupy any more space - they are hard-linked into each newsgroup directory.
If you do not understand this, please talk to your newsmaster.
Leafnode can be found at http://www.leafnode.org/