This very short article was the most informative so far on the March towards passive scanning (that already started in NY and LA train stations). The TSA is expanding its testing. The scanner is made by Thruvision:
"[Thruvision's Kevin] Gramer said, the passive terahertz technology reads the energy emitted by a person, similar to thermal imaging used in night-vision goggles.
"'It’s 100% passive. There is no radiation coming out of our device.'"
This seems like an improvement on the privacy front, but it really depends on how the technology is used. I have more to say on this, but I'll save it for another time.