If i'm not wrong, there are no cookies that kept you logged in. I think that browsers (IE,FireFox,etc) put user:password@domain.com when you give them the order of remember user and pass. I think themaPoster do the same.
And themaPoster is really fast loggin in a forum. I don't get why do you ask for this feature, but i am not the creator
.
But i hope my info were useful to you. Remember. When you want to direct log in on a site and you don't trust in your browser, you just do like this: user:password@poster.freddy.it
It's something like that. So, there is no reason to loggin in on a forum with this.