What you are doing is establishing a booster gap. This becomes the highest form of resistance in the circuit and takes the spark energy and changes it. The conditions that exist, most likely fouled plugs or rich mixture have made the spark required very low voltage (with a longer duration to disapate the energy). By creating a booster gap you have artificially upped the spark voltage (shorter duration) probably on that car to around 25kv (it was probably around 5kv if left to the conditions that existed).
The extra spark voltage is overcoming the mixture or plug problem.
Bosch Master, ASE Master, L1
33 years MB technician