This sterospecific reaction proceeds through a bridged cation like the bromonium ion where `H` replaces `Br`. The intermediate may actually be a `pi` -complex. In the absence of a solvent that can satbilise a free `R^(oplus), Br^(Ө)` attacks the protonated complex from the opposite side resulting in trans (anti) addition.
In the case of a polar solvent such as `H_2 O`,the protonated complex collapses to the free `R^(oplus)`, which now reacts from both sides (faces) and gives both `cis-` and trans-additions.
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