First, focus the subject with looking through the viewfinder.
After that, turn the milled mirror-up knob in the direction of the arrow, and at the same time release the shutter.
At this point, the mirror inside springs up without being released the shutter.
And again depressing the shutter release button releases the shutter and exposes the film.

Note: The mirror-up function decreases the shock caused by its spring-up movement through the shutter releasing.