It’s an energy weapon, commonly known as the “Pain Ray,” that turns electricity into millimeter wave radio frequency
When the signal goes out over radio to shoot me, there’s no warning — no flash, no smell, no sound, no round. Suddenly my chest and neck feel like they’ve been exposed to a blast furnace, with a sting thrown in for good measure. I’m getting blasted with 12 joules of energy per square centimeter, in a fairly concentrated blast diameter. I last maybe two seconds of curiosity before my body takes the controls and yanks me out of the way of the beam.
Loree says the boot-up time on the Pain Ray is “sixteen hours.” So if the system is at a dead stop on a base and, say, the locals protest the burning of a Koran, guards at the entry points won’t be burning anyone. The Directorate says that in a realistic deployment, the Active Denial System will be kept in ready mode — that is, loudly humming as its fuel tanks power it, or hooked up to a base’s generator. But that makes it a gas guzzler, at a time when the military’s trying to reduce its expensive fuel costs.

“That’s something we’ve really got to look hard at, how do we make the system as efficient as possible,” says Marine Col. Tracy Tafolla, the head of the Joint Non-Lethal Weapons Directorate, “to make sure that we’re not running a lot of fuel.”

Another problem is less technological and more fundamental. In 2010, Gen. Stanley McChrystal, then the commander of U.S. troops in Afghanistan, sent the Pain Ray back to the States after a deployment of mere weeks. His reasoning: it was too great a propaganda boon to the Taliban, who’d say the U.S. was microwaving Afghans, giving them cancer, making them sterile, and so forth.
More here: Video: I Got Blasted by the Pentagon's Pain Ray -- Twice | Danger Room | Wired.com