Dropping Out : Dropping out occurs when the computer's CPU does not have to both perform operations and maintain output to the display device. Dropping out is indicated when the portions or all of the display momentary go blank(background color).
Snow : Snow occurs when the computer is not able to maintain operations and the display. The effect is similar to what is seen on a standard television that is receiving a weak broadcast signal.
In the case of flicker, however, there is software designed to prevent the computer from sending anything to the display unless there is time to go through a full scan pattern.