How is it physically possible that there is no voltage drop across two pins of this DC jack?
No voltage is measured with the switched Pin because it is no longer connected with anything. In this diagram used for illustration purposes Pin 3, the Switched Pin, is physically disconnected when the plug is inserted.




Pin 2 is physically moved by the barrel. Its normally made of a spring metal that can bend and return to its normal position when the plug is removed. The 3rd picture is a Phone connector with, but the concept is the same in a DC barrel jack.

The voltage seen is 9v, with +9V on the center pin/Pin 1, and 0V/Ground reference on pin 2 (for a center positive adapter).