There are three possible upgrades that can be called from the version check process; the World of Warcraft launcher, the World of Warcraft game client, and the World of Warcraft Public Test Realm client. Not all of them are necessarily updated at the same time, and some of them may have more frequent updates than others.
The Update Process
If a patch update is relatively large, Blizzard may stagger the update to download small increments over a number of days to stage the pre-launch update on the end user hard drive. Once Blizzard releases the final patch update, the client update program gathers together all of the previously downloaded data chunks and performs the patch update with the aggregated information.
This "chunked" download process allows players over slower dial-up connections to methodically gather the information they'll need to update their World of Warcraft client without a significant download delay. Because the download has occurred little-by-little, dial-up users won't have to wait for a large patch to be downloaded in a single sitting.
The configuration settings for downloads can be found in the Launcher under "Options."
The "Download while playing World of Warcraft" option allows Blizzard Downloader to run while a user is online inside the World of Warcraft universe. This option would obviously only apply if an incremental patch is downloaded for a future release. World of Warcraft players on high-bandwidth Internet connections will probably not notice the downloading during gameplay, but dial-up users will probably want to avoid this option. The default is disabled.
The "Download after exiting World of Warcraft" is the default for all users. This option isn't required, but the on-screen description explains that you'll be forced into downloading the entire patch eventually.
An interesting selection is the "Don't throttle background download" option. By default, the Blizzard Downloader only uses a portion of your available bandwidth. If this option is selected, the Downloader will use all available bandwidth. If you're not planning on using your Internet connection for other purposes, this option may provide a faster patch download when using high speed links.