There are paints, fabrics and wallboards made for the same purpose now. The real neat trick here is it's a paint that can be painted on existing structures and controlled with a switch.
I certainly don't know for a fact how they are doing it, but I would imagine they are not flipping a switch to turn the paint on or off, they have the paint on the outside that blocks the signals, then they hae an antennae outside, a repeater that acts like s cell phone tower, and an antennae inside and they simply turn the repeater on and off to control acessability inside...