IMHO, may be something like this:
If meter show 1/100s, F4 @ ISO 200, mirror goes up once and sensor took 3 exposure of
1) 1/300s, F4, ISO 300,
2) 1/300s, F4, ISO 600, and
3) 1/300s, F4, ISO 1600.
Combine all this 3 images to give a single shot HDR RAW. To the user, he took 1 shot, but actually the camera took 3 bracketed shots.
Have a nice day.