While shooting on aperture priority on most of the modern DSLRs the exposure calculation or metering at Evaluative metering is biased towards the focusing point selected. This is good as most of the time you focus at the subject, the most important element in your frame and obviously, you would like to have the best or the ideal exposure over this area.
This works quite well in backlit subjects as the camera itself compensates for the darker subject.
However at times, you may want the subject to appear as a silhouette or let it appear darker for creative reasons. While shooting movies with DSLRs at aperture priority, you may notice the exposure change too fast and so many times, not let you register absolute darks, leading to part of the frame go over exposed. This typically happens when the frame includes a large portion of black. As the camera is making this area grey, it tends to turn a well lit normal area too bright and over exposes this part of the frame since the focus is on the black. As the focus shifts to the well lit area, the exposure also changes but the change is not very fast. In such situations, one may note the ideal exposure and switch over to the manual mode so that the exposure does not change with the change in focus.