For Nikon D300 and D700 photographers (moi included), the photo above shown is what I call the "winding motor" of our respective DSLRs share. It's responsible for cocking our shutter curtains and pushing the mirror of our viewfinder down after the brief "blackout" you see during exposure-taking. This mechanism has moving parts (obviously) and, hence, good for just around 150,000 actuations. But it will show signs of "wear and tear" at around 100,000 actuations by failing to push down your viewfinders' mirror-every now and then. Other DSLR models differ in winding motors' durabilities. As the wear continues, you will experience mirror blackouts more often. When this occurs, I usually turn my D300 off to force the mirror to return, or hit the AF/AE lock in tandem with the shutter release button. What a hassle! It can cost you up to Php14,000 for this winding gear's replacement, so be sparing during your trigger-happy moments FYI.