Sange is the term used when a Hero sacrifices a part of her body for Mankai.
Sange is done almost immediately after Mankai. The user is unable to use the part that was sacrificed, and the Hero Form is changed to accommodate the lost part if needed (e.g. Ribbons to move, sensors to hear).
Sange was taken away when Yuna made the decision to sacrifice herself to defeat the Vertex, and all body usages were given back.
On the daily life of a hero, the Sange effects them tremendously. In cases like Fu, both wore a certain article of clothing to help them adapt better. Sange has almost no effect in battle, however. The form utilizes a different form, such as extended ribbons or sensors to hear, if the Sange would get in the way during battle.