In his account, competition between mid-vowel diphthongization (e.g., s[e]ntir 'to feel', s [je]nto 'I feel') and non-diphthongization (e.g., p[e]dir 'to ask ', p[i]do 'I ask') leads to paradigmatic gaps in lexemes for which the applicability of diphthongization has low reliability (e.g., abolir 'to abolish, *ab [we]lo, *ab [o]lo 'I abolish').
this approach both overpredicts and underpredicts the existence of gaps crosslinguistically.