How does low impedance measurement reveals ghost voltage?
What is the logic behind using low impedance and voltage being
  disappeared?


The "ghost voltage" you refer to is a real voltage and although high in amplitude it is potentially very weak due to it being sourced via a high impedance. A high input impedance measurement device (o-scope, multimeter etc.) can measure this voltage with little or no obvious reduction in the amplitude of the voltage but, a low impedance device will try and take current through the inherent high impedance in series with the voltage, and the voltage will collapse in amplitude, sometimes just to a few milli volt.

It's not a ghost voltage in that someone or something hasn't created it following a death; it's just a voltage that is in-series with a very high impedance.

For instance you can capacitively couple to AC voltages in house wiring and this coupling might only be 50 pF. An oscilloscope will display the voltage on your body and tell you that the voltage is a few volts peak to peak yet, 50 pF has an impedance at 50 Hz of 63.7 Mohm. This will form a potential divider with your o-scope input (usually 10 Mohms) and the 120 VAC might drop to around 10 or 20 volts. Prior to connecting the multimeter the voltage would be higher.