Abgt declension of счастье

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.

The only falling vowel I can propose in "счастье" is и before е which in contemporary Russian almost always replaced by ь. The only exception is Genitive Plural счастий.

Though it does not make quoted stuff from your post clearer for me.

To find out declensions this is enough.

The gen. pl. ущелий, свиней and so on.

купанье беганье катанье also have some similarity to счастье, in the endings patterns.