  Twisting by a G-torsor an object endowed with an action of a group G is a
classical tool. For instance one finds in the paragraph 5.3 of the book
"cohomologie galoisienne" by Serre, the description of the "op\'eration de
torsion" in a particular context. The aim of this note is to give a
formalization of this twisting operation as general as possible in the
algebraic geometric framework and to present a few applications. We will focus
in particular to the application to the problem of specialization of covers
addressed by P. D\`ebes and al. in a series of papers.
