Cernit Magic Mix is a plasticizer that is used in the production of polymer clay, therefore you will get a clay that is even more flexible and resistant to breaking after baking. It will not alter the color of the clay. It can be used to thin out liquid clay as well. You can also use it to erase fingerprints and other irregularities on the surface of clay before baking.
Cernit Magic mix can be used to recover clay which has hardened (but not baked), just break up the clay into smaller pieces and put it into a container with a small amount of Magic Mix. Add a little bit by bit until you can work the clay as you like.
If the clay is just stiffer than you prefer, Cernit Soft Mix is also an option.