The central topic is this question: is a given $k$-étale algebra $∏_lE_l/k$ the specialization of a given $k$-cover $f:X→ B$ at some point $t_0𝟄 B(k)$? Our main tool is a {\it twisting lemma} that reduces the problem to finding $k$-rational points on a certain $k$-variety. Previous forms of this twisting lemma are generalized and unified. New applications are given: a Grunwald form of Hilbert's irreducibility theorem over number fields, a non-Galois variant of the Tchebotarev theorem for function fields over finite fields, some general specialization properties of covers over PAC or ample fields.