Answer
This reaction happens thorough dehydrohalogenation. The product is shown below and the stereochemistry is retained.
Work Step by Step
There is formation of carbene with one bromine on it. Carbenes are formed by reactions of halogenated compounds with bases. There is dehydrohalogenation in which the hydrogen and the halogen are lost from the carbon atom. In this case the reagent is $CH_2Br_2$, and thus one hydrogen and one halogen is lost to give the following product.
