When plugged into a device, the USB Killer rapidly charges its capacitors from the USB power lines. When the device is charged, -200VDC is discharged over the data lines of the host device. This charge/discharge cycle is repeated many times per second, until the USB Killer is removed. Simply put: used on unprotected equipment, the USB Killer instantly and permanently disables unprotected hardware.

Source: USB Killer – ESD Tester to test and disable USB devices