I'm very confused about the differences between these methods. From my textbook, it states that the Huckel method only takes into account the pi bonding interaction while the Extended Huckel will take into account all the valence electrons. But what exactly does this do?

Because to my knowledge the S, Pz, Px, and Py orbitals are all orthogonal to one another and wouldn't mix anyways. Is the only thing accounted for by the extended theory shielding effects?