
When two edges are already marked with "x" in Lshape as in this figure, let's think about the vertex placed diagonally from the Lshape corner. 


A line comes from either top or left edge to the star marked vertex,
so it extends to one of the remaining edge (right/bottom).
This part is same as the case of "3" (rule of drawing a line out of a corner of "3" when two lines are already connected in Lshape). 

Drawing a line out of a corner of "1" when two "x" are already marked in Lshape  

This is the opposite of the rule of drawing a line to a corner of "1".
This is not so useful by itself, but is very helpful in the following cases:

