If you take a photo with a long lens "over 100 mm or so" and you have the camera ,light like a *ist, on a light tripod when the mirror flips up this hole mess can vibrate. If the shutter speed in the range of lets say .5 to 1.5 seconds this can make motion blur in your photo. The exact speeds depend on your setup. If the shutter speed is fast it will be all over before you have to much movement. If the shutter speed is slow it will dampen out and be to small a percentage of the total to show any blur.
When you use the "mirror clean" option this is just so you can have access to the sensor for cleaning. Pushing the shutter button tells the camera to stop. This is not the same as using the "mirror lock up". When you use mirror lock up the camera will flip up the mirror about one second before opening the shutter so that the camera/tripod can stop moving before the shutter opens.
Battle at Zama