That is indeed how it works Greg. I use my plug inserted into the metal "boot" to check and set my timing. I don't "ground" the plug to the engine "block". The is "no" connection between the engine and CDI box on any of my systems other than the one created by the shield around the high tension lead. The voltage goes out through the plug wire "core", through the plug, across the gap and returns back to the CDI through the outer metal braid or "shield".