Capacitor over the Battery
It works for punch (been there, tried that), but that's just a side effect.
The reason a capacitor is put on the battery is because is helps the Fet's of your esc to "shut" more easily. When the fet is open, a large current passes through it, when it shuts, the current won't stop that easily. It can't go back into the battery, since that's the source from which it eminates, so it's fed into the cap.
The reason you want this, is that the Fet is very inefficient when it is opening or closing. In either fully open or fully closed position it doesn't burn away much energy, but during the closing it does. The large current going through the fet when it closes would cause much heating and even inhibit the fet closing..
The trouble is, that a normal capacitor isn't quick enough to charge and discharge at the frequency's used by today's esc's. If you want to try one, get one from Novak or LRP.