A simple explanation of how a plate array in a HHO electrolysis device also acts as a capacitor. When the unit is switched off the plates hold a slowly dissipating charge, and the plates still produce small amounts of gas. A capacitor of this config holds energy in the electric field between the plates.The voltage meter shows the voltage of the cell after the current has been disconnected.

This is by it's very nature a capacitor but it also converts to a hydroxy production cell.