An intermediate is likely for amylase and a-1,6 glucosidase.
The a-1,6 bond and the a-1,4 bond are important for the formation of a bond to one part of the chain.
A second nucleophile would attack and release the chain.
Chapter 21 would be a terminal C4-OH group of glycogen to receive the transferred tri-glucose unit, while debranching would use a water molecule as the second nucleophile to release a free glucose monomer.
Without a-amylase treatment, the glycogen will have a high molecular weight and remain at the top of the gel.
The true weight of the glycogenin will be reflected in the migration distance.