As I see it, PSK is the simplest way to prevent cycles, SSK allows more possibilities, and NSSK allows even more, but how does this strictly prevent cycles? Does it have to do with two-pass endings?
To be honnest I don't know.
My guess would be it is to allow as much as possible. (so that the superko rule prevents as little as possible.
you can see the progression
I think Repeat of moves is the most allowing superko rule that is possible while it still will make sure a game will always come to an end. (but it can take much longer)
There is a discussion if it all makes even can make a theoretical difference.
Where can I find this discussion?
maybe sbest to start a discussion in the rules forum on life in 19 x 19 i have seen some old discussions and private emails about it.
But the best way is to start a new one... (see you there :)
started a discussion http://www.lifein19x19.com/forum/viewtopic.php?f=45&t=1479