[John]

Here's a little more info on how this works.

Whenever $forumid (and often $forum[forumid]) has a value, it runs a query which checks to see if security is enabled for that particular forum. (The security field in the "forum" table.)

If security is enabled, it will check to see if a cookie exists for that forum containing the password. If there's no cookie with a correct password, the user will be directed to a page where they can enter the password.

The password they enter is then checked, and if it matches the one in the database a cookie will be deployed. (And the whole process will start again, but this time the user will be forwarded to the forumdisplay page.)

The same applies for the code in showthread.php.