Pass As Ko Threat
An interesting question is whether passes can serve as ko threats. In other words, can a player recapture in a ko if he did not play a ko threat on the board, just passed his relevant turn instead?
Suppose it's black's turn here and he takes the ko. This is useless by normal go logic: black is still dead. White cannot take back the ko immediately, but can come back anytime later and capture black if he wants.
But some ko rules don't take passes into account. In their case, if white doesn't even want to play a "dummy" threat (because the board is full like here, or because of some scoring issues), the situation can get tricky: white may even get locked out of the ko forever. This is particularly an issue for Superko rules, since their nature is to look for repetitions of the board position only.
AGA rules are an example of a rule set which would not allow white to capture.
Note that when white passes and takes back the ko after, the position does seem to repeat, with black to move again. But the game doesn't enter a perpetual cycle, as the situation is not the same: it is now black who cannot take back the ko immediately, so white will have time to finish the capture.
Bill: This is a popular way of putting it, but it is a misstatement. Any board play, not just a ko threat, typically lifts a ko or superko ban. The question is rather, does a pass act like a regular play? For Ing, passes lift ko bans, and that is why he defined a pass as a kind of play.
However, if only a pass lifts a ko ban, as with the hypothetical play of the Japanese '89 rules, calling a pass a ko threat is apropos.