The Flatten Subsystem tool automatically flattens a Simulink subsystem, that is, it moves the subsystem contents up one level, reconnects the signals, and removes the subsystem block. This tool is useful for making quick transformations to a model by automating the tedious task of manually copying and connecting the many elements contained in a subsystem.
Note: This functionality has been since included in Matlab/Simulink 2014a as the "Expand Subsystem" operation. This tool is still beneficial because it works for earlier versions of Matlab/Simulink, and it handles more types of subsystems (e.g. atomic subsystems).
For installation and other information, please see the User Guide.