Hydrochloric acid parafuchsin reacts with Sodium Nitrite to form diazonium salt (six-azo-pararosaniline). α-Naphthyl Acetate is decomposed by esterase and produces α-naphthol, which shall combine with diazonium salt to form red brown precipitate in cytoplasm. This stain has no specificity against esterase; therefore, it is called "Non-specific Esterase Stain".