Right in the middle are two L-shaped pieces forming a cross. They are the prongs. The bit sticking down on the left is the trigger, before it's pulled. To load, the gun is opened up, an elastic band is hooked around the end of the barrel, and the other end of it is hooked around the top prong. Then the prongs are pushed round anti-clockwise. There is another elastic band on the trigger so when the prongs rotate and push it back it snaps back into postion. Another is loaded onto the new top prong, etc. It can take a lot of elastic bands.
In this photo the trigger has been pulled, allowing the prongs to rotate, being pulled by the force of all the elastic bands on them (you'll just have to use your imagination). The prongs reach this position, the top elastic band fires off, and the prongs can rotate no further. Then when the trigger is released the prongs can turn another 45 degrees ready to fire again.