Charlie Howard (dpreview, Panasonic forum, 19 February 2004)...
There are springs under the side-rails of the hotshoe. When the one on the right is pushed down, the camera assumes an external flash is present and turned on. The spring under the left rail is just a spring; the camera doesn't sense anything on that side.
It doesn't matter whether the flash is actually turned on, or even whether you've actually attached a flash: if you push a toothpick in there, the camera will try to use it.
You can test this yourself by attaching a flash and leaving it off, or by using anything that depresses the right-side spring (I used the probe of a voltmeter).
The center pin is one contact and the rest of the hotshoe is the other contact of an electrical circuit. Until you fully depress the shutter button, that circuit is open, so half-pressing the shutter doesn't signal anything to an attached flash. When you fully depress the button, the circuit closes momentarily, triggering the flash (if there is one and it's turned on), then the circuit reopens.
{Picture below `Spring Lake Estuary, Spring' by Charlie Howard. Taken with FZ10 (without a flash)}
tranworld (dpreview, Panasonic forum, 21 February 2004)...
If you look under the spring, you will see a black pin, this is a switch and the pin will be pushed down when the spring is depressed.
