http://www.innovationsforum-energiewende.de/wp-content/uploa...
To put this in simple terms:
Normally the plant runs at a constant rate of energy consumption, due to restrictions in how the production process works.
This means sometimes there will be a deficit of available electricity, other times there will be a surplus.
They plan to change their production process such that they can:
1. soak up the surplus usefully when it is present
and
2. "give back" energy to the grid by reducing their own consumption when there is a deficit
They never feed back any energy, they just make it so more energy is available on the grid by reducing their own consumption.
The difficult part here is that there is a certain engineering effort necessary to make this possible in the first place without disrupting the production process.