(2S,3R)-3-Bromobutan-2-ol is warmed with concentrated HBr. The OH group is lost (as water) with anchimeric assistance by the neighbouring bromine, giving a bridged bromonium ion that is opened by bromide, and 2,3-dibromobutane is formed. The product is

The neighbouring Br attacks C2 from the side opposite to the leaving water (first inversion), and external bromide opens the bridged ion at the same carbon from the back (second inversion), so C2 is retained and the product is (2S,3R), which is the meso compound. Direct SN2 inversion at C2 alone would instead have given the chiral (2R,3R) isomer. Priorities at each centre are unchanged, so the descriptors do not shift.