They add DC, through a current limiting resistor, on to the inner conductor of the coax. It's not really "phantom powering" in the normal sense as it's routinely applied to balanced circuits via two matched 6.8 K resistors but the manufacturer, (Sennheiser possibly but I've forgotten who it was) was calling it "phantom". Definitely "phantom", not "T-powering" as commonly used in the film industry.Hmmm... how do you phantom-power a two-conductor jack?
Toodleoo!
Ron Hebbard.