From the back, with the arms swung rear-ward, it can be clearly seen how the cantilevers at the end of each arm keep the elastic taut. When the arms are swung forward again, there is a moment where the elastic doesn't move much while a moving platform raises the next brick into place. The cantilevers thus make it possible to catch the brick - as the arms move outward, the elastic is drawn tight. The trigger mechanism keeps the brick from firing.
