It is a self-cocking shutter. Pressing the shutter release lever first cocks the shutter and then releases it.
Your shutter needs service.
Oh, and by the way, you really should unscrew a cell and check how far the diaphragm opens. Since the shutter blades won't open fully, the rear cell is the one to unscrew 'cos the diaphragm is behind the shutter blades. You're in for an unpleasant surprise; wide open the diaphragm opens to only around 25 mm.