As has been correctly pointed out to me by @xavierluthi, this technique doesn't require you to have a phone. It uses the perturbation to radio frequency signals that *your body* unavoidably creates just be moving in RF signals. The WiFi is used to generate a RF signal for you to affect, if there is none. But given that many parts of the world are now already covered with WiFi, it raises even further questions regarding privacy implications.
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