Corona discharges will be seen at any larger bleed off points. No visible coronas are not a guarantee of no leakage however. Here is an equivalent circuit schematic of the ignition system and a regular and Nology spark plug wire. The circuit shows why the Nology wires capacitors produce a stronger spark. The Nology wires capacitors are formed by simply wrapping a wire braid around a segment of the wire and then grounding it.
Construction is very similar to coaxial cable.
Nology uses a carbon fiber type center conductor, but it has a much lower resistance than the other fiber wires tested. The braid also acts as a shield to prevent EM radiation. The capacitance is evenly distributed along the length of the braid and the center conductors resistance is distributed along it's length. The equivalent circuit above is a theoretical representation of this. In an ideal simulation there would be an infinite number of very small capacitors and resistors which would all add their effect to define the model of the wire.