Catalyzes the formation of branch points in alpha-glucans by cleavage of an alpha-1,4 glycosidic bond and subsequent transfer of the cleaved-off oligosaccharide to a new alpha-1,6 position. The branch chain-length distribution of the reaction products shows degree of polymerization (DP) of 3 to 13, with two local maxima at DP 7 and DP 11. Exhibits an alpha-retaining catalytic mechanism. Is involved in glycogen biosynthesis. Shows a secondary activity, i.e. the hydrolysis of the substrate, being 4% of the total activity. Can use amylose as substrate but not alpha-1,4-linked oligosaccharides of 2-7 glucose residues, beta-cyclodextrin, 6-O-glucosyl-beta-cyclodextrin and 6-O-maltosyl-beta-cyclodextrin. Is not able to branch amylopectin further, it only hydrolyzes amylopectin. Thus, displays preference for linear and long substrates (amylose) over branched structures (amylopectin). Transfers a segment of a (1->4)-alpha-D-glucan chain to a primary hydroxy group in a similar glucan chain. Optimum pH is 6.5. Optimum temperature is 65 degrees Celsius. Remains fully active following incubation for 1 hour at 80 degrees Celsius. Glycan biosynthesis; glycogen biosynthesis. Belongs to the glycosyl hydrolase 57 family.