These fricatives are called palate-alveolar, which can be taken to mean that their place of articulation is partly palatal, partly alveolar.The tongue is in contact with an area slightly further back than that for s, z. If you make s then sh, you should be able feel your tongue move backwards. The air escapes through a passage along the center of the tongue, as in s and z, but the passage is a little wider. Most speakers of PR have rounded lips for sh and ch, and this is an important difference between these consonants and s and z.