A website can't detect exactly when your visit starts and ends. The only way a website knows you're online is when you tell your browser to load something new from it.
Every time you click on a new page, SMW Central sees that you're requesting a page, and it sees the request came from you, so it marks you as online.
if you do nothing for 5 minutes, the site just guesses that you're offline, because it can't tell for sure. It can't tell if you've left the site, or if you're still looking at the site and just haven't loaded anything new. 5 minutes is an arbitrary limit, but it works well enough.