This tools expands the spark gap of any massive electrode aviation plug safely and efficently -- with the key word being safely... In the past when you over-gapped a plug, and wanted to expand it back out, you would resort to using a screwdriver, dental pick, or whatever else was on hand. But this was almost a sure way to do damage to the plug.
These devices measure the change in inductance in the magneto coil when the points open and close. Simply shorting the clip leads together will not indicate a point closing, therefore eliminating any false reading caused by loss of contact to the magneto grounding terminal. To use, simply remove one plug from each cylinder.