| Hi,
I use these all the time. They're mostly useful for switching more than one thing on/off with the plug.
Here's the deal:
There is a "chassis" lug, which connects to the housing.
This, obviously, is ground.
There are two diagonal rows of lugs along two opposite edges of the jack.
These are usually numbered 1,2,3 and 5,6,7, in little molded-in numbers on the plastic casing.
These lugs do not connect electrically to the cord in any way.
Each of these rows is a SPDT switch. When the plug is out, the middle lug is connected to one of the outer lugs. Inserting the plug breaks that, and connects the middle lug to its other neighbor.
For example, in a set numbered 1/2/3, the cases would be:
Plug out: 2 & 3 connected, 2 & 1 not
Plug in: 2 & 1 connected, 2 & 3 not
So, to use this for a battery switch, you could connect the battery hot to #2, and the +9v power input for the preamp to #1.
The other set works the same way.
I've found that on most 9-pins, plugging in the cord connects the middle lugs to the pins closest to the edge, but check that with your meter if you have one.
The other lugs, usually marked 4 and 8, are the usual tip and ring terminals for the cord itself. Tip is usually #4.
You can use the ring terminal (usually #8, also usually closest to the ground/chassis lug) for battery switching in the usual manner (connect battery black/ground wire to it so it gets connected to ground when you plug in).
These plugs are very useful and actually pretty reliable, I've found. We use them on the Veillettes with magnetic/piezo combos, since our blending preamp uses a bipolar dual battery setup, so each battery has to be switched independently.
Good luck.
Cheers,
Martin | 
