What is the stereospecific reaction explain with example ?

The chemical reaction in which a stereoselective reactant gives another stereoselective product called Stereospecific reaction for example trans addition of HBr with with but-2-ene give 92% meso-product but cis but-2ene give almost l00%(dl)/ racemic mixture. The sterespecificity of the addition can be explained in terms of a bridged structure in analogy to the bridged bromonium ion in the ionic addition of HBr to alkenes.