http://egami.blog.so-net.ne.jp/2011-02-12#english
Canon made a half mirror called "pellicle". Sony made a half mirror and called it "translucent". Whilst both are fixed, Nikon have thought about to make it movable!
So, what is the point? I just wonder.. When the mirror can be moved, it does not need to be semi-transparent. When it is semi-transparent, it does not necessary to be moving. Why tries to make both at the same time? To get the best of both worlds or maybe actually, just to get the worse things of both?? (Just read the last comparison table in the above link to find out more..)