From Sakata's Go, vol. 6: How to Count, p. 12.
has just taken the ko. Black to play will save his large group by play in the center, leaving behind a two stage ko in the bottom left corner.
At first glance this looks like a three stage ko, with 3 legs, but if White takes , the local temperature will drop, leaving a sente ko. White's capture effectively wins the hotter (two stage) ko. The apparent third leg is not really part of that ko.
(Actually, that may not be correct, given the interaction with the bottom position. This will take some more study. In any event, Sakata called this a nidan ko, so perhaps the above explanation is what he had in mind.)