Sony Mobile published a nice explanation of the "HDR Video" mode in its stacked Exmor RS sensors inside the flagship Xperia Z smartphone:
How HDR video recording works
To achieve true HDR video recording, the advanced Exmor RS camera sensor in Xperia Z changes the exposure every two lines of pixels. This creates a spatially varying exposure (SVE) array image, which is then used to make an image that is perfectly exposed in both dark and light areas.
To simplify the concept, for each frame the camera shoots one picture with two different exposure levels at the same time, one light and one dark. Then, an intelligent software algorithm is used to combine (synthesize) the two shots into one picture, pixel by pixel. This allows you to see the details of both the dark and the bright parts of the picture.
HDR for still images
With HDR for still images, the camera quickly captures the image twice with different exposure levels and stitches the two shots together to create one optimised photo. This way, HDR lets you capture clear photos even against strong backlight. HDR is automatically activated when Superior Auto detects scenes with strong backlight.