Maybe Sony sensor, so you do not need dual ISO.
Maybe, but more likely than not, it's just clever processing work with data from the sensor. It does have a DIGIC 6 on it I believe, which means it should be fast enough to bracket a frame at two ISO (maybe only 24p?) so that it can average them out? Hopefully it will allow you to choose two ISO's, and not have built in values that can't be changed. I'm really looking forward to this, I might upgrade to this from my T3i.
Do any other Canon DSLRs or camcorders have "HDR" movie to get an idea of what this will look like? Maybe other non cannon cameras?