Forgive this newbie question but I am new to large format photography, so I'm unfamiliar with some of its components. I have, for a 4x5 camera, a Fujinon W 6.3 lens in a Seiko shutter. I'm clear about setting the aperture and shutter speeds (B up to 1/500), and cocking and firing the shutter, but I'm foxed by a lever that lives in a small slot to the left of the red dart on the speed selector ring when the shutter is set to B. It moves a little to the left, then returns when released. It doesn't seem to do anything, so I have no idea of its purpose. Can anyone help? With thanks in advance.