Complex sentence: There was a fair amount of compensatory lengthening involved where a loss of consonantal gemination lengthened the preceding vowel, whence the transformation of عشّ ('nest') into عوش.
Split into simpler sentences: There was a fair amount of compensatory lengthening involved where a loss of consonantal gemination lengthened the preceding vowel. Whence the transformation of عشّ ('nest') into عوش.