In a perfect world, that solder tab should not move. Problem is, people overheat it when soldering to it, which melts the plastic surrounding it, making it loose. The solder tab is connected to the end of the secondary winding of the coil, consisting of many turns of hair-thin wire. If it is a good connection, electricity can flow with no resistance. But if it is a poor connection, electricity can still flow, but will jump (arc) across the poor connection gap. That is why it can still produce a spark. However, each time it arcs, it burns away at that fine wire, eventually making such a wide gap that the electricity can no longer jump the gap.
The resistance (Ohms) across the poor connection is what shows up as an abnormally high meter reading.
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