Rebuild the Fulltext catalog

Question

I have a Database (80GB) that is able to store attachments. These attachments are full text indexed. For some reasons I had to rebuild the fulltext catalog and this took 30 hours.

Our business users noticed while rebuilding the new catalog as some searches didn't work anymore as expected, but some did.

I was wondering how SQL Server proceeds while rebuilding the fulltext catalog.

Does it delete all index and then start rebuilding it? Or is there some intelligent logic behind it, like delete just a part of it piece-by-piece?