why bimolecular elimination reactions are stereospecific?

I know the mechanism of this reaction but it is stated that these are stereospecific in nature . why do we call these stereospecific explain with example.

• The hydrogen being abstracted by the base, and the leaving group must be anti-periplanar. This requirement determines the stereochemistry unambiguously , and thus the reaction is stereospecific. – getafix Aug 17 '16 at 20:27