It is a one-step reaction. A base donates an electron pair to an alpha-hydrogen, forms with it a bond, pushes electrons of the C-H bond (and breaking it) to the C-C bond (and foring a C=C bond). The leaving group is pushed away with its electron pair, breaking the C-Br bond and releasing Br^-.

The reaction proceeds through the most stable anti-staggered transition state, which makes it an example of anti-elimination.