As far as I know you don't need to reverse polarity in this technique, in fact is not really good. But the trick with the welder machine is true, Ni-CD batteries suffer from crystalline formations over the time which reduces the cell capacity (a.k.a. Memory Effect), the high current over the cells in a pulsed way can broke those crystals and revival the battery.
If you don't have a welder machine you can do this with less than 5 bucks in your own house:

For those that are not familiarized with diodes, the white strip is pointed to the RIGHT in the picture. Put the resistor underwater if you want, it gets real hot. Make pulses with the wires just like the guy in the video, just take care to not short the battery
