It's not intended to make hydrogen. It generates an ionic frequency that the car can be tuned to. Not sure if it's the same but I've heard this sort of thing referred to as orgone energy or zero-point energy. From what I understand it can only be used in an aluminum or alloy block, like the v8 rover he first experimented on.

In April 2006, American experimenter Bill Williams, who tested this technology, was death-threatened by two unidentified persons who knew all about him and his family, etc... This event has, of course, sparked a resurgence of interest in the Joe Cell