That's because they're shooting explosions. With firecrackers and a fish tank, they're getting up close and personal with the gorgeous spheres of light and oxygen which billow out from an explosion under the water. With a Phantom camera, they're cranking up the speed (and pushing down that resolution) to really see every tiny detail of these pop.
Check it out.
As ever, it's gorgeous stuff - which makes it not so much of a surprise that these folks were slow motion consultants on the criminally underrated Dredd movie. Well Gav was, we're not really sure what Dan does.
The water being flung from the surface of the tank might be the most visually stunning part of the video, very much like the flecks which were a part of the high frame rate photography of Dredd, but the burst of light and that perfectly spherical bubble are pretty spectacular too.