By way of making excuses for the Bronica S2a mirror slap which is known to cause children to weep and induce seizures in small animals...
-- Mirror slides DOWN so it's lying on the floor of the camera, facing shiny-side-up! You'll notice that all four corners of the mirror have roller-tipped arms on them; these move in curved slots in body sides to guide the mirror in the proper path. The mirror is yanked down by a fabric ribbon cemented to its back; at firing, this ribbon rolls up around a roller running crossways in the bottom of the camera.
-- Having shiny mirror facing up in the bottom of the camera would cause crazy reflections, no? So, a metal flap, hinged at its bottom edge and lying just in front of the shutter curtains, lies down on top of the mirror to block it. There's no separate linkage to control this flap -- it's just lightly spring-loaded and normally is pressed back by the top edge of the main mirror; when the mirror lies down, the spring-loading flips down the flap on top of it.
-- Now the only problem is the focusing screen above the camera throat. On most SLRs, the mirror flips up and covers this; with a down-sliding mirror, it's wide open to let light fog your film. So, there's a little fabric 'window shade' on a roller running crossways at the TOP front edge of the mirror box; when the mirror lies down, two cords attached to its upper corners and running around pulleys unroll this 'window shade' so it covers the finder opening. After exposure, as the mirror slides back up, the curtain winds back up on its spring-loaded roller.
That's some pretty freakin' amazing engineering if you ask me. So many points of failure, but they just keep going.