Kim has made his USB Killer by combining camera flash parts with AA battery and made it such that the USB pendrive will deliver 300 volts when plugged into an unsuspecting computer. It’s effectively the same as a power surge, but the difference is that most power supplies have protection against those. The inbuilt USB buses in computer/laptop don’t offer the same kind of resistance, and consequently relay the energy to the rest of the computer, instantly killing it. Therefore, just be careful next time before inserting any untrusted USB pendrive that does not belong to you into your laptop. You could not only lose all the important files and data stored in your computer, but also lose your laptop.