How the spring works?
Well, to set the trap, one pushes the springs down, spreads the jaws flat, and a flat pin is laid over a jaw and held in a notch under the pan. The animal presses down on and moves the pan, knocking the pin that is holding the jaw down. In the picture posted by chrispinnh, one can see the flat pin front and center.
So, the springs are relaxed when the jaws are closed, the springs are pushed down and angry, the jaws are opened and held open by the pin, once the pin is knocked out of it's place, there is nothing to hold the jaws down, the spring comes up and closes the jaws.