Removes maltotriose and maltotetraose chains that are attached by 1,6-alpha-linkage to the limit dextrin main chain, generating a debranched limit dextrin (By similarity). Hydrolyzes the alpha-1,6-glycosidic linkages in amylopectin while does not hydrolyze the alpha-1,4-glycosidic linkages in amylose. Native glycogen is a poor substrate (PubMed:11554733). Hydrolysis of (1->6)-alpha-D-glucosidic linkages to branches with degrees of polymerization of three or four glucose residues in limit dextrin. Slightly activated by Ca(2+). Inhibited by divalent cations such as Zn(2+), Cu(2+), Fe(2+), Mg(2+), Mn(2+), but only slightly inhibited by EDTA. Optimum pH is 7. Optimum temperature is 40 degrees Celsius. Glycan degradation; glycogen degradation. Belongs to the glycosyl hydrolase 13 family. Was originally (PubMed:11554733) identified as an isoamylase despite of its little activity on glycogen, but was renamed as glycogen debranching enzyme due to the characterization of the flanking gene coding for the glycogen branching enzyme (PubMed:12480526).