[Trystan]

It is based on your current cookie and the last time the server registers you as having logged in. Everything that was posted before the last time stamp your account is tagged with is counted as read. Between time stamps the site cookie marks which threads you have read.

so, if you clear your cookies before your account gets time stamped, all threads that have posts after the last stamp show up as read. (just tested this)

I don't know how long the server waits between stamps though.