Are you sure? I think at 1/320sec the curtain should open completely. Sequentially opening of the curtain should be first at less than 1/1000sec? (please correct me if I´m wrong)
Consider yourself corrected.
The time needed for opening the curtain is only a little less than what the manufacturer quotes as the minimum shutter speed suitable for use with electronic flash. For the Canon 5D Mark II, the User's Manual says that "The camera can synchronize with non-Canon compact flash units at 1/200 second and slower speeds."
If the shutter could open significantly faster than that, then the manufacturer would quote a faster sync speed.
Even if there is a time when the sensor is totally open the exposure does not start and ends at the same time over the entire sensor.
I don´t belive that,
JH is correct, though I'm not sure how his explanation relates to his picture. [Edit
: Ah, I see. On the monitor I was using, it appeared that the bottom part of the image was completely black. On another monitor, or with a levels adjust, I see that the bottom part of the image is indeed dimly exposed, and with a different color balance.]
Exposure begins when light begins to hit the sensor, after the sensor has been electronically cleared.
With continuous illumination and mechanical shutter opening, the electronic clearing happens while the shutter is still closed. In this case exposure will begin as soon as the shutter curtain moves from in front of each portion of the sensor. The side where opening occurs first will begin its exposure about 1/200 second before the side where opening occurs last.
With continuous illumination and EFSC, the electronic clearing is done incrementally while the mechanical shutter is already open. It proceeds sequentially across the sensor mimicking the motion of the mechanical shutter. Again, the side where clearing occurs first will begin its exposure about 1/200 second before the side where clearing occurs last.
In both cases, exposure ends when the light is cut off by the mechanical shutter moving in front of the sensor.