The debranching enzymes are a set of two specialized proteins that play a critical role in glycogenolysis. When a glycogen molecule has been broken down by glycogen phosphorylase to the point that there are less than five glucose subunits on either side of a branching point, the debranching enzyme is needed to continue the breakdown process.
The first debranching enzyme, glucosyltransferase, moves three glucose subunits from one 'branch' to the other 'branch', lengthening one at the expense of the other. This leaves a single glucose subunit at the branching point.
The second enzyme, glucosidase, then cleaves the single glucose subunit, thereby eliminating the branching point.
Glycogen phosphorylase is then free to continue the breakdown of the glycogen by removing glucose subunits from the end of the (now unbranched) chain.