The taper cavity acts and performs like a resonant charging choke. He talks of the mixture as acting like Micro capacitors thru the cavity.
The negative SS voltage should be a reflection of what what the positive charge is, NOT ground. (atomic restructuring because of the water there between, in the metal) The SS will polarize to a negative or positive voltage and hold so much charge.
Remember that the SS negative part of the injector is "acting" like a resonant charging choke, because of the atomic structure and shape. So think of the SS part as if it was a coiled choke.