The second palatalization of velars is a direct consequence of the monophthongization of diphthongs, or more precisely, the change *aj > ē.
Compare: The intermediary /dz/ has been preserved only in the oldest Old Church Slavonic canon monuments, Lechitic languages, and the Ohrid dialects of Macedonian.
In Russian, Slovak and (in nouns) Slovene, the results of the second palatalization were later removed at morpheme boundaries (i.e. before inflectional endings) due to paradigmatic leveling by analogy.
Compare: For Northwest Russian varieties (Novgorod, Pskov), according to Zaliznyak,[5] the second palatalization did not take place at all (E.g. Pskovian kev', but OESl.
According to others, however, “these seemingly unchanged velars were actually palatalized dentals in both the ancient artifacts and in the modern varieties (so such k- would actually be [ť]).