From Journal of Applied Physiology, 1969 27(5): page 765.
“O2 deficit and debt. A collection bag is connect to each outlet (A, B, C, D). Initially when the valve is in the position shown, exhaled gas flows from the mouthpiece and into the bag on the outlet which is in position A. When this collection period ends the outer rim is rotated so that outlet A is in position B, and an empty bag is in position A. When the next collection period begins a third empty bag is in position A, the second collection bag is in position B and the first bag is in position C and ready for sampling. When the fourth collection period begins the above sequence is repeated, and the first bag is rotated into position D and emptied into the Tissot. The first bag is ready to be refilled when turned to position A at the beginning of the 5th gas collection.