At Hosshin-ji, noted for its severe sesshin, on the fourth night and every night thereafter, at eight, there is a unique discipline for combatting the seductive visions of bed which begin to tug at the fatigued, faltering mind at this hour. With the clang of the large zendo bell there is a sudden outburst of “Mu-ing” by all who are striving with this koan. At first weak and uncertain, this collective howl gathers depth. force and momentum under the energetic prodding of the free-swinging kyosaku of the godo and his aides, who yell: “Voice Mu from the hara not from the throat!” When these cries of ‘Mu!” reach a crescendo of deep bellows, as they eventually do, suddenly they are turned off by the clanging of the same bell, usually about thirty minute later.  Now silent zazen resumes but the air has become electric.

… And during the severest yearly sesshin the rohatsu, in December, which commemorates the Buddha’s enlightenment, yaza (zazen after 9pm.) is the rule each night. The last day of sesshin, especially if it has been preceded by all-night zazen, tends to become anticlimactic. The roshi therefore admonishes everyone that this is the most critical day of all and that to relax now when the mind has reached its peak of concentration is in effect to throw away six days of solid effort. On this seventh day the kyosaku and its companion yelling are dispensed with - as though the godo were saying “I can do no more, it is now entirely up to you” - and after (or because of) six days of periodic bedlam this day of silent yet dynamic zazen is frequently the most rewarding.

Before the formal close of a sesshin the roshi, addressing everyone, will say in effect:

A sincere effort at sesshin is never wasted even though it does not terminate in enlightenment. Getting kensho can be compared to a person’s hitting the bull’s-eye on the hundredth shot. Who can say that the ninety-nine misses are unrelated to the final success?” 

The Three Pillars of Zen compiled and edited by Philip Kapleau pub. Harper and Row 1967)