Note: I am not asking about about seseo vs distinción, /s/ vs /θ/, or how "ce/ci/z" is pronounced in Spain vs Latin America. This question is about the letter s and the sound /s/, as in ser, español, es, and of course "ce/ci/z" in dialects that lack distinción.
Someone already asked this question, but none of the answers there (including the accepted one) actually answer it - instead they talk about /s/ vs /θ/. My question is how is /s/ pronounced in Spain that's different from how it's pronounced in other Spanish-speaking areas, and different from how it's pronounced in English?
I've read that the Iberian Spanish "s" is pronounced "halfway between the english s (/s/) and the english sh /ʃ/", and also that it's "retracted" (/s̺/), or that it's "shorter" than the English /s/ - but none of those explanations are very helpful, nor is the Wikipedia entry on retracted alveolar fricatives.
Let me describe as precisely as possible how I pronounce /s/ and /ʃ/ (I'm a native speaker of British English):
/s/: My tongue is spread wide; the edges of my tongue are below my molars. My tongue is mostly flat and is forward in my mouth; the tip is slightly below and behind my top teeth, and air escapes through a gap a couple of millimetres wide between my tongue and my incisors.
/ʃ/: My tongue is much narrower and taller - it's narrower than my top teeth all the way to the back. The tip is maybe a centimetre behind my top teeth, and my tongue points slightly upwards before curling down again at the tip. Air escapes across and down the tip of my tongue behind my top gums.
Can someone give me a similarly precise description for the Iberian Spanish /s̺/? And is the Latin American /s/ exactly the same as the English /s/ I've described above?