A former Japanese student sent me the link to "
Dangerous English".
(Caveat Emptor: This is an example that is commonly used by the taste-challenged instructor in Japan, nonetheless.) The s/sh and z/zh distinction is, indeed, difficult for some learners. A survey of youtube videos reveals a few typical teaching strategies:
(1) Smile on 's'.
(2) Round lips or pucker on 'sh'.
(3) Hiss like a snake on 's'.
(4) Remember how your parents told you to be quiet on 'sh,' etc.
A few take a shot at talking about (5) tongue "grooving" or
(6) "elevating at the back."
Most involve just (7) listen and repeat or listen and repeat s-l-o-w-l-y--
(8) along with practice saying the sounds in different word position and contexts.
My "favorite" example of the latter is
this one. She is at least pleasurable to watch . . . She does a list of words that includes a few that I'm not even sure SHE knows the meaning and collocation of. One of them (which I assume that she DOES know, however) is "circumcision," a word that includes that potentially "dangerous" s/sh alternation.
Here is a EHIEP haptic protocol for s/sh/z/zh:
(1) For 's' or 'z' the lips should just be relaxed.
(2) For 'sh' or 'zh' the lips should just be slightly rounded (not the exaggerated smile and pucker of the "Dangerous English" instructor!)
(3) For 's' or 'z' begin by placing right forefinger horizontally in front of the mouth so that you can feel the air coming out on your middle knuckle as you do the sound. (Imagine the forefinger is a sharp knife?)
(4) As the sound is being produced,
with both eyes staying focused on the right forefinger, slowly move the right hand up to about hair-line level.
(5) For 'sh/zh' begin in the same position and then slowly lower right hand down to just below the chin.
(6) In the case of 'z' or 'zh,' also place left forefinger on the vocal cords as the sound is produced.
What that does, among other things, is direct the air stream through the upper teeth on 's/z' and through the lower teeth on 'sh/zh.'
So for the word, "circumcisions:"
(1) Go up slowly twice (on "cir" and "ci"),
(2) Go down with left forefinger on the vocal cords (on "si'), and then
(3) Go back up on final "s" (with finger still on vocal cords.)
Can you get the "felt sense" of that technique? It does have its "ups and downs," of course, but it almost always "sits" well with learners!