>>10147973the second line is just an ad hoc definition , the substitution that goes from to 1 is defined in the first line. And the definition in the second line is just analogous to single variable calculus, where there is also a difference between two applications of the same function at very small distances.
In this case the distance is the hypotenuse of the triangle that goes from (x,y) to (x, y+bh), to (x+ah,y+bh) (back to (x,y)), where a and b are parameters that change in relation to the definition in line 1. (as if the triangle was circumscribed in a unit circle)