The method was to build a corner reflector out of three orthogonal conducting sheets, but to connect one of the sheets to ground via a PIN diode switch. By turning the diode on and off, the reflection coefficient of the corner reflector could be modulated, in turn modulating the reflected signal.
I'm not sure what the outcome was, though it must have gotten to a certain point of success, since I've memories of a car driving around with a corner reflector on it.
The same student was experimenting with using a plasma as the modulated reflector, but that one ended when he connected the wrong end of the vacuum pump to his gear, accidentally pressurised it, and blew it up!
Not meaning to be critical, rather just want to clarify that our modern-day gigawatt-scale energy grid yields an enormous 15 orders of magnitude more electrical power than the microwatt-scale "harvesting" of TV signals can. That's an enormous power spectrum, shown to have a newly useful application (batteryless mesh networking) at microwatt scale.
;-)
This project is trying to figure out what the equivalent of an rfid tag looks like with that power model. They come up with a scheme of being able to absorb versus reflect the ambient signal to communicate between unpowered tags. Maybe the most productive way to think about this is as a step into the larger research area of how to effectively harvest energy and do useful stuff with devices that don't need batteries.