When making a stop cut across the face of a piece of wood that is wider the the length of the blade on your knife, you cannot "rock" the blade across the entire width of the wood. You still do not want to cause the knife blade to come off the edge of the wood. If you attempted to use the knife like a pencil to make the stop cut, it may indeed come off the edge on the wood; and perhaps into the hand holding the wood. The two photos illustrate one way to avoid the knife blade coming off the edge of the wood.
......then while holding the blade steady, "rock" the wood into the blade, rather than "rocking" the knife. Continue this "rocking" motion until the wood is moved so that the blade is over the edge. This method requires some steady pressure on the sharp knife to achieve the desired stop cut.
When this stop cut is made in this manner, it's almost impossible to cut yourself.