Think of sound waves reflecting off a wall. As soon as the first wavefront hits, it is sent back. With this thing, you would send soundwaves at the wall, and i

Think of sound waves reflecting off a wall. As soon as the first wavefront hits, it is sent back. With this thing, you would send soundwaves at the wall, and it would store the energy in the sound for a short period before sending it back. It looks like it compresses the time interval for energy release as well. So you'd play a little Mozart at normal volume at the wall, and then after (say) 1 second it would send a cacaphony of noise back at you. It also looks like they can control what direction the wall sends sound back in. And it looks like the period in which they can store energy is very limited. As for "can control when the energy is released" - they control it by controlling when the incoming signal stops. They have to do a lot of control on the incoming signals for this to work. They mention future work to build such a device with photons. I'd hazard a guess that then you could use this to build a fancy phased array radar analogue.