When you're working with skinned characters, it's common to need a special shape to fix a skin deformation problem when a character is in a certain pose. Deformation problems typically occur at a character's articulation points, such as the shoulder, underarm, knee, and groin areas.
You can fix these problems with a technique known as pose space deformation (PSD). Using the Pose Editor, Maya uses a pose interpolation algorithm that reads joint transformations (poses) in order to drive corrective blend shapes. The poses within an "interpolator node" are all normalized against each other and are mutually exclusive from other poses.