How did the Verge make the tablets fall down in the new Nexus 7 video review?

At the 3:48 second mark in the above video, the new Nexus 7 and iPad mini are shown standing next to each other and within a few seconds they both fall down together. I saw another tablet in the background that probably was keeping them propped up. What I want to know is how did they make the tablets fall on cue? It wasn't obvious in the video how they pulled this off. Any guesses?