The reason power save works. A station wakes, reads one bit addressed to its association ID, and goes back to sleep if it is clear — so a device can sleep through almost every beacon interval and still never miss a frame.
Subfields
DTIM Count — How many beacons remain until the next DTIM.
DTIM Period — How many beacon intervals lie between DTIMs.
Bitmap Control — A traffic indicator for group-addressed frames, plus an offset into the bitmap that follows.
Partial Virtual Bitmap — One bit per association identifier, set when that station has traffic buffered.