How is plunge defined?

The definition of plunge is the downward angle measured from the horizontal plane.  It is positive down.  You can type in a negative value of plunge if you want an upward trend to the out-of-plane axis (into the computer screen extrusion direction).  However, you can also mirror the excavation and give a positive plunge with 180 degrees added to the trend. 

How the 2D tunnel section is actually transformed is that first the section is rotated about the vertical axis until the out-of-plane axis is in the trend direction.  Then the section is rotated downwards (or upwards if it's negative) by the plunge angle.