Twistings, crossed coproducts and Hopf-Galois coextensions
  Let $H$ be a Hopf algebra. Ju and Cai introduced the notion of twisting of an $H$-module coalgebra. In this note, we study the relationship between twistings, crossed coproducts and Hopf-Galois coextensions. In particular, we show that a twisting of an $H$-Galois coextension remains $H$-Galois if the twisting is invertible. 
S. Caenepeel, Dingguo Wang, and Yanxin Wang