The Electronic Engine Ionizer is just of a couple of pieces of wire molded to rubber blocks, or "capacitor blocks", as the manufacturer claims.
There aren't any capacitors in them, however. The rubber blocks clip onto the spark plug wires near the plugs, and supposedly carry the "corona charge" from one cylinder's plug wire to the electrodes of the other plugs. This charge is supposed to "cause a partial breakdown in the larger hydrocarbon molecules in all the non-firing cylinders, resulting in increased combustion efficiency”.
Yeah. Normally, engineers would work to prevent cross-coupling between spark plug wires to prevent cross firing between cylinders. The Engine Ionizer seems engineered to promote crossfire.
Verdict: In a test where the magazine Popular Mechanics did to test whether it actually works or not, their test truck showed a 15 horsepower loss. On top of that, the Ionizer melted and dripped onto the exhaust manifold, which then resulted in 2-foot tall flames. They had nothing to say about it's fuel saving properties, naturally.