via TechCrunch:
The arrow scene, where the sandcliff stage becomes a vertical climbing wall, is a variation of the old carvinal knife throwing trick. When each archer fires his dummy arrow, a matching “arrow” is propelled out of the walls’s surface, giving the illusion that it has hit its target. A little puff of “dust” completes the illusion. After that, though, things get more complicated as performers climb up and down the arrows while more arrows land and others disappear. To ensure the performers’ safety, each arrow (or “peg” as they’re called) contains a sensor which prevents it from shooting outwards if someone is standing or laying on top of it (“otherwise they’d be impaled, which would be bad”). Likewise, the pegs can only contract with less than 20lbs of pressure, so if a performer is still hanging off one, it stays put. And if all that wasn’t enough, the wall also contains a bank of “emergency pegs” which can be shot out to form an escape ladder/bridge if anything goes wrong.