In Mishnah Yuma 1:7,
ביקש להתנמנם, פרחי כהונה מכין לפניו באצבע צרידה
If he [the Cohen Gadol] sought to doze, the sprouting youths of the priesthood would strike before him with a צרידה finger.
[left צרידה untranslated because not sure what it means]
It seems from Kehati's explanation that this is the familiar action of snapping one's fingers, done using the middle finger, thumb, and palm. But Bartenura seems to say it involves the first finger: how would that work--how would a sound be made?