How does HDR movie work? The new DSLR camera released by Canon, 760D, has capability of recording HDR movie. My question is how does this work? In still photography, when setting the camera to HDR, the camera will take 3 shots (the mirror will flip 3 times), with different shutter speed to adjust the exposure and then process it to combine into 1 picture.

For movie, definitely the mirror will need to be flipped all the time while simultaneously need to record at different exposure level. So how does the camera manage to handle this?
At a guess, it will be a similar technology to HDRx from RED or Magic Lantern - 2 frames instead of one recorded for every single video frame.


Reference 1: http://www.red.com/learn/red-101/hdrx-high-dynamic-range-video
Reference 2: http://cgi.tutsplus.com/tutorials/a-simple-way-to-shoot-hdr-video-footage-using-magic-lantern--ae-20993


In Magic Lantern every frame is recorded at 2 ISO values, in the case of the RED camera, two frames are captured instead of one with different shutter speeds with the second frame "2-6 stops faster".
The selection of ISO/"stops faster" is carried out by the user in both case prior to recording.

All that has to be done then, is to blend the resulting "dual frames" into a single video frame for an HDR video.
Also, keep in mind that you do not need 3 frames for an HDR - you just want to cover a greater range than a single exposure can cover, this can be 2 frames or 200 frames. For video it is 2.