Involved in the biosynthesis of the mercapturic acid derivative diterpene cyslabdan A, a potentiator of the beta-lactam antibiotic imipenem (PubMed:26507838). Catalyzes the conversion of (+)-copalyl diphosphate to yield labda-8(17),12(E),14-triene (biformene) (PubMed:26507838, PubMed:26814669). (+)-copalyl diphosphate = (12E)-labda-8(17),12,14-triene + diphosphate Binds 3 Mg(2+) ions per subunit. The Asp-Asp-Xaa-Xaa-Xaa-Glu (DDXXXE) and Asn-Xaa-Xaa-Xaa-Ser-Xaa-Xaa-Xaa-Glu (NSE) motifs are important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.