The description is found in a paper about the German fricatives https://www.degruyter.com/view/j/labphon.2014.5.issue-1/lp-2014-0005/lp-2014-0005.xml
How do the three sound differ from each other in a spectrogram? Do diachronic sound changes from [ʃ] to [ç] or reverse generally have an intermediate stage of [ɕ]?
Subjectively (if can be generalized to other people, acoustically) I perceive the sound is something between [s] and [ʃ], though the description is "...have simultaneous alveolar and palatal articulation". Do different people have different perception of articulation place of the sound [ɕ]?