Yoma, Chapter 1, Mishnah 7. In this section we continue the discussion of how they kept the Kohen Gadol awake on the night of Yom Kippur, so that he would not fall asleep and come to have a seminal emission.
The words of the Mishnah and their explanation:
"Bikesh l'hisnamneim" - when the Kohen Gadol wanted to sleep and began to doze off.
"Pirchei kehunah makin lefanav b'tzrusa" - the young kohanim would snap their fingers in front of him. They are called 'pirchei kehunah' because "perach" means to sprout, and their beards were only beginning to grow.
"V'omrim lo: Ishi Kohen Gadol, amod v'hafeig achas al haritzpah" - "Ishi Kohen Gadol" - my master, the Kohen Gadol; "amod" - stand up; and walk on the floor, since walking helps to shake off fatigue and sleep. This walking was on a cold marble floor, and the cold rouses a person and drives away his sleep.
"U'ma'asikin oso ad shiyagia z'man hashechitah" - they would keep him occupied: singing before him and doing various things that kept him awake, until the time arrived to bring the tamid offering.
In summary: This Mishnah teaches the practical methods they used to keep the Kohen Gadol alert on the night of Yom Kippur: the young kohanim snapping their fingers, getting him on his feet to walk on the cold marble floor, and keeping him occupied with singing and various activities - all until the time arrived for the slaughtering of the tamid.
Once the time for the slaughtering arrived, the process itself began, as we will see in the next Mishnah.