Connector difficult to unplug

The OBDII connector on the vehicle has a locking tab that retains the UltraGauge OBDII connector, as can be seen here:

OBDII Connectors

Depending on the design of the vehicle's connector, it can take as much as 9lbs of force to unplug the connector.   On other vehicles it may offer very little resistance.

The amount of force required is directly related to the slope of the back side of the tab.  The steeper the slope, the greater the force required.

There is no trick to unplugging the connector, just grasp the body and pull, but pull cautiously.  Never pull on the cable as damage will occur.