This is only effective with the first line of quantitization blocks. Let us look at an example.
Example 1 a1 : (Exist x) (f(x)) by ...;
Example 2 a1 : (All x) (Exist y) (g(x,y))by ...;...............
Therefore, if attaching a part of a2 in front of formula a3 and parenthesizing it makes formula a1, it means that inference of FLW is correct.
Thus, FLW is an abbreviation of follow.