Outdoor Flash with no ambient light

I have been watching Bert Stephani's videos on youtube and this guy is amazing. Using one or two flashes outdoors in the daylight, he achieves dark backgrounds behind his models. How does one do this? In this video

http://www.youtube.com …8PGxL6urg&featu​re=related

I mentions that if he raises his shutter speed to 1/250 he gets the dark background. With some much ambient light, how is this done?

Notice that the position of the flash was not far from the subject...the female assistant raised her arm to cue the photographer to the model passing at that instant. If the flash had a Guide Number of 150 (e.g. 580EX at 50mm FF setting), it would have been easy to get an f/11 aperture as the correct exposure (GN150 / 11 = 14'). The sky was heavily clouded...maybe ISO 100 is 1/100 f/8, so that basic exposure is already -1EV under the flash. Change shutter speed to 1/250 and now the area lit by ambient only is -2.5EV compared to the model...near black!

It would not have been possible if they were under sunny skies and not the clouded overcast day in Belgium.