PCB mounted jack pinout?

[add4]

I am playing with PCB mounted jacks and i don't get how the middle switchable pin is behaving the jack is connected.

http://www.taydaelectronics.com/hardware/dc-power/dc-power-jack-2-1mm-barrel-type-pcb-mount.html

On the tayda pic, it is the 'side' jack.
With no jack plugged in, my multimeter says it's connected to the 'middle' pin, which should be ground then.

When the jack is plugged in, i read 9v between the rear jack and the middle jack .. that  is then + and ground respectively.
but i read 0v between the side pin and ground pin, and 0v between the side pin, and the + pin ... which makes me go ... HUH?

the resistance between the side pin and both ground and + pins is infinite.

Can someone explain me what's happening?
:p

[jkokura]

It could be that you have a switching jack. That pin may be connected to either ground or to the tip when you have nothing plugged in, but plugging something in lifts the switch and then it connects to nothing, or switches from tip to ground, or from ground to tip, or whatever it's setup for.

Jacob