Two-tiered relexification in Yiddish

The book claims that Yiddish was created when Judaized Sorbs first relexified their language to High German between the 9th-12th centuries; by the 15th century, the descendants of the Judaized Khazars also relexified their Kiev-Polessian (northern Ukrainian and southern Belarusian) speech to Yiddish and German, Yiddish thus uses a mixed West-East Slavic grammar and suggests that converted Khazars were a major component in the Ashkenazic ethnogenesis.