This sounds like the experience that Ananda had acc to the canon. On the eve of the first council Ananda was still only a stream-winner and was in the position of not being able to attend the council as he was not an arahant. He was doing the walking meditation for hours, right up until the dawn of the council, still not finding the answer. Eventually he gave up and lay down for a rest, still being mindful of his actions. He gained arahantship the moment he lay down. Same experience here?
Although the pattern is similar, in this case it was not the realization of arahantship for Ajahn Chah. Rather, it was a direct experience of some of the deeper samadhi states, namely arupa (formless) jhana. However, it certainly meant progress for him on the path.