This video demonstrates the use of the extract deltas plugin in maya on osx and windows. (Sorry, there is no audio available).
Usage: Pose your model in which a corrective shape should be applied. Select the geometry and run the duplication command. This creates a copy of the skin, removes the intermediate shape and unlocks the translation channels. Use this copy to model your corrective shape. Then select the skin and the copy, make sure the extractDeltas plugin is loaded, and run the extraction command. This will take the bound base mesh and applies the modeled corrections to a copy of it. The result can be used as a corrective blendshape for the actual skin.
The plugin can be found here: braverabbit.de/extractdeltas/
If you are looking for a way to streamline your blend shape and corrective shape workflow check out SHAPES, a blend shape editor for Maya which combines a variety of tools for your everyday work. braverabbit.de/shapes/