It is done only on NiCad cells where one cell in a pack is not showing voltage and won't take any charge. It is done by touching a 12v source + to + and - to - , too the cell momentarily like in 1/2 second. Often it will revive the cell but sometimes only temporarilly but if the pack is then left on a float charger, the cell is generally trustworthy as long as the pack meets the 70-80% capacity test.