Unless the settings have been changed since I last touched it, it works like this:
If the user has 35 or more accesses within the last minute, a warning is issued in place of the actual page, but the user is not banned. This should wake up the user to slow down their pace. A margin of 10 accesses is still allowed for the same timeframe in case the user was just busy opening some tabs.
If the user has 45 or more accesses within the last minute, a 6-hour ban is enacted.
If the user accesses the trap link (blockme.cgi) once, a 1-minute ban is enacted immediately.
If the user accesses the trap link while a 1-minute ban is active, a 1-hour ban is enacted immediately.
If the user accesses the trap link while a 1-hour ban is active, a 36-hour ban is enacted immediately.
If the user's agent is a known mass-downloading program and the access concerns autogenerated pages (i.e. not submission files etc), a warning is issued in place of the actual page, but the user is not banned.
If the user accesses a referrer-restricted page (some particularly heavy autogenerated pages
such as diffs) without being referred by the main site, a warning is issued in place of the actual page, but the user is not banned.
Access blocks are stacked, i.e. acquiring a 1-minute ban while a 6-hour ban is active does not shorten the ban to 1 minute, nor does a warning remove the ban.
I have originally developed these measures based on the known-good-in-practice model developed at Sensei's Library:
http://senseis.xmp.net/?AccessBlocked