Features of the voiceless alveolo-palatal fricative:
Its manner of articulation is sibilantfricative, which means it is generally produced by channeling air flow along a groove in the back of the tongue up to the place of articulation, at which point it is focused against the sharp edge of the nearly clenched teeth, causing high-frequency turbulence.
Its phonation is voiceless, which means it is produced without vibrations of the vocal cords. In some languages the vocal cords are actively separated, so it is always voiceless; in others the cords are lax, so that it may take on the voicing of adjacent sounds.
It is an oral consonant, which means air is allowed to escape through the mouth only.
It is a central consonant, which means it is produced by directing the airstream along the center of the tongue, rather than to the sides.
Allophonic variation of /ʃ/. Contrasts with other sibilants only in onset. Argued both to be laminal[ʃ], and generally produced "in the middle of the hard palate", same of fellow alveolo-palatal [l̠ʲ] and [n̠ʲ], and further palatalized than Italian post-alveolars. Found in coda mainly before fricative, coronal and palatalized consonants in Brazil. See Portuguese phonology
Landau, Ernestina; Lončarić, Mijo; Horga, Damir; Škarić, Ivo (1999), "Croatian", Handbook of the International Phonetic Association: A guide to the use of the International Phonetic Alphabet, Cambridge: Cambridge University Press, pp. 66–69, ISBN0-521-65236-7