Note that shorting the output of a microphone (whether dynamic or phantom powered) will cause no harm to 99.9% of microphones, because they require protection from the phantom power anyway. A high value is necessary, because most mics have a fairly low output impedance, generally less than 200 ohms. So, if we can't apply a direct short, it needs to be done using capacitance. That's a large signal, even if the mic output is high. The voltages on Pins 2 and 3 only need to differ by a small amount (say 1%) to generate a signal of up to 480mV when the two leads are shorted. The level from a microphone varies widely, being anything from a few millivolts to almost a volt - depending on the source being picked up by the mic. We normally expect that the DC voltage on each line of the mic cable will be the same with respect to Pin 1 (ground), however, component tolerances and other factors will combine to make this expectation unrealistic. Shure has a published circuit that is the basis for the circuit shown here (not that there are many alternatives).
Dynamic mics can simply be shorted out (between pins 2 and 3 of the XLR connector), and while that approach might be ok with phantom powered (P48) supplies, there may be some noise if the current in each signal wire is not identical.
It's not a particularly common requirement, but I have been asked about it, and decided that what little information is on-line is somewhat lacking in details and/ or reasoning.
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