It tries to catch spammers at registration time, before they ever post. When a user registers, their username, email address and registration IP address are checked against the StopForumSpam database, and then put in a queue for me to review if any of those things match. I look at the queue each morning and approve or reject manually.
If something gets through the net and the team or I clean up the spammer using the spam tools, the data is then sent to StopForumSpam to improve their database, to let it get better at catching potentially malicious registrations.
In the year or so that we've been using it (since we moved to the new forum software), I think I've rejected just one legitimate account by mistake (the user registered again, which is possible), yet I've been able to successfully stop hundreds, if not thousands at this point. There are usually 5-10 a day that get caught in the net.