The mirror lock up feature is enabled. This eliminates the vibration caused by the mirror flip when the shot is taken. This slight vibration causes a slight amount of blurring. Cameras use a mirror that reflects the image up into the viewfinder and during an exposure this mirror must flip up momentarily for the light to reach the film or exposure. The mirror lock up feature allows you to put the mirror up before you take the picture and keeps it there until after the picture is taken. The value of this mirror lock up is in crisper images. It is essential for telephoto, close ups and landscapes.
In AV mode I enable this mirror lock up. Now I must depress the remote twice. The first one locks up the mirror and the second one takes the image.