They modify their getwork server to report less rejects/stales. There's no mystery there. I can do the same here if it'll make you feel better, though. I can, in fact, using their methods, give you 0 rejects.
PPS is the only reward type that trick
wouldn't work for. For a full rebuttal, see
https://bitcointalk.org/index.php?topic=33586.msg465303#msg465303