SIM Agar is a semi-solid medium specifically designed for the triple biochemical identification of Enterobacteriaceae. It utilizes the hydrogen sulfide color reaction mechanism (sulfur-producing bacteria such as Salmonella form black iron sulfide precipitates), indole color reaction (indole-producing bacteria like Escherichia coli turn red after adding Kovacs reagent), and motility diffusion interpretation (motile bacteria spread outward along the puncture line, while non-motile bacteria grow only along the inoculation line).