When an algorithm is applied, the smooth shading no longer functions as before, since some new vertices are not "shared", e.g. a square with 2 triangles can be represented with
v 0 0
v 0 1
v 1 0
v 1 1
f 0 1 3
f 3 2 0
This representation allows the smooth shading to behave as usual.
The follow does not:
v 0 0
v 0 1
v 1 1
v 1 1
v 1 0
v 0 0
f 0 1 2
f 3 4 5