My thinking is that Rossi might be under-driving some HID lamps in order to obtain unstable operation that can give back difficult to measure, noisy voltage spikes. Perhaps he believes that he's obtaining excess electrical power in this way. This could be similar to what he tried to accomplish in the Stockholm demonstration with a hand-made setup.
As far as I understand, commercially-available electronic HID ballasts generally try to maximize lamp endurance and to obtain stable operation, so they would have to be altered in some way.