There is only a resistor and three series LED's on that strip, look at the picture. There's no room anywhere else for a diode or bridge, since the back is adhesive, so there is no rectifier. I suspect they're depending on the three series LED's reverse voltage specification being sufficient for 18V AC. Truthfully, I wouldn't run these on more than 12V AC or DC, since you're running the LED's beyond the 20ma specification at any higher voltage. If you look closely at the picture, you'll see they even printed 12V DC right on the strip and included the +/- connection polarity. They're not really suited for AC, but I suspect those bozo's connected them and they didn't die in the 10 second test.