how dose FastDL Work

FastDL ("sv_downloadurl") simply specifies where the extra content can be fetched from:

Once the game client tries to connect to the game server, the game server will tell the game client "Hi Client. I'm Server, and I would like you to fetch the following list of data: maps/random_map.bsp, sound/some/random/sound.mp3"

If the sv_downloadurl isn't set (empty!), the list of content will be downloaded directly from your game server (assuming sv_allowdownload is set to 1), which can be pretty slow and limited in speeds.