Comment by d--b

There are sensors in the device. They're activated by a battery that starts when you put the thing on and clip the zipper. You have to charge the battery every 3 weeks or so. I've got one and have been using it for a month. So far I didn't fall, and the stuff hasn't been triggered by any of my jumps on and off the sidewalk. It's much more comfortable than a standard helmet. It's very expensive though.