[Plugin Simple Tagging] – how does add tag work?

How exactly does the “Add Tag” feature work? You say that it “lets you add one or more new tags to all posts which match any of the tags given” – does this mean that it scans ALL the posts you’ve made for the input-word, and then tags them automatically if that word is found in the scanned posts?

The reason I’m asking is because I’ve got over 1000 posts, and I would hate to mess anything up. I’d like to tag all the posts to make a tag cloud, and that’s why I chose your great plug-in. How long would this scan take on this many posts do you think?

Thanks in advance