"The USB Killer is shockingly simple in its operation. As soon as you plug it in, a DC-to-DC converter starts drawing power from the host system and storing electricity in its bank of capacitors (the square-shaped components). When the capacitors reach a potential of -220V, the device dumps all of that electricity into the USB data lines, most likely frying whatever is on the other end. If the host doesn't just roll over and die, the USB stick does the charge-discharge process again and again until it sizzles.
Since the USB Killer has gone on sale, it has been used to fry laptops (including an old ThinkPad and a brand new MacBook Pro), an Xbox One, the new Google Pixel phone, and some cars (infotainment units, rather than whole cars... for now). Notably, some devices fare better than others, and there's a range of possible outcomes—the USB Killer doesn't just nuke everything completely."