This is basically the exact opposite of the traditional white-background portrait, as described at How can I get a pure white background in studio photography?, or a modification of What do I need to get photos with a unifom black background? Once you understand those, this is relatively easy.

Raphael explains one possible model, but I think it's actually simpler than that. The ambient, natural light is such that at some reasonably fast shutter speed (¹⁄₁₀₀th or maybe a little faster) and a moderate-wide aperture, there's a dark blue sky and a largely black ground. So, there's no need for a trailing-sync exposure or for the subjects to move quickly.

There's a flash basically aligned with the camera and a little up; maybe even on-camera. It puts a huge amount of light on the relatively close subjects, and spills over to light the ground. You can see the shadow on the ground behind the anti-silhouettes, where they block that spill.

Then, there's probably some post-processing to adjust the curves to bring up the sky a bit (the "lumpy" look is indicative of trying to pull sky detail out of the shadows), and maybe also at the high end to make sure the whites are mostly clipped.