Be careful not to push the pushpin into your finger, as opposed to the small notch that you've just uncovered.

Once the spring hinge extends outward with the first pushpin, use a second pushpin to keep the hole in the "extended" position:

There will be a small notch revealed once the spring hinge extends outwards. Wedge the second pushpin into that notch so that the hinge doesn't go back inwards.

I sadly only have two hands, so I was not able to show how to wedge the pushpin and take the photo.