Trying to understand the right declension of счастье I go to the dictionary where I find the following explanation (the word follows the model 6: ""6* Subpattern with falling vowel. This subpattern embraces the substantivs with radical (graphic) in falling vowel. (Phonetically they are radicals in [i] preceded by a falling vowel). In tonic / stressed position this falling vowel is "e"; in unstressed position it is "и".""
The model followed is: ущелье, ущелья, ущелью, ...
Which falling vowel? Where is it? Graphic? Phoneticcally? Could you explain it in words that an Economist, so am I, could understand? Thank you.