USB Kill

The USB Kill 3.0 is a testing device created to test USB ports against power surge attacks. The USB Kill 3.0 tests your device's resistance against this attack.

The USB Kill collects power from the USB power lines (5V, 1 - 3A) until it reaches ~ -240V, upon which it discharges the stored voltage into the USB data lines. 
This charge / discharge cycle is very rapid and happens multiple times per second. 
The process of rapid discharging will continue while the device is plugged in, or the device can no longer discharge - that is, the circuit in the host machine is broken.