Q:

How is "plug-in auto detection" realized with this circuit?

This question is specific for ACT2802X series 5V/2.5A Backup Battery Pack Manager IC.
On page 15, there is a schematic that provides a solution for auto plug-in detection. However, I cannot understand how this is achieved? What is the thing that comes out from the USB connector's shield in the schematic? I cannot also find a USB connector with "plugged" output.

PB pin is described as:

Push button input, connect a push button from this pin to AGND,
  internally pulled up  by a 3MΩresistor to battery. PB starts up boost
  converter if HZ pin is grounded and  VIN is not present. For
  ACT2802B/ACT2802C, if this pin is pushed for 3s, flashlight is 
  enabled.

...

For ACT2802QL-T/ ACT2802-T1026, When HZ=0, if PB is pulled low  more
  than 100ms, boost converter is enabled; For  ACT2802B/ACT2802C, when
  HZ=0, if PB is pulled  low more than 30ms, boost converter is enabled.

PB High Input Voltage = 0.9V (typical)
PB Low Input Voltage = 0.75V (typical)

A:

Typically, the shield is connected "somewhere" to the ground connection.  Either at this connector or at the other end of the cable.  In either instance, plugging in the cables will short the case/shield to ground which is equivalent to closing the PB switch.

