They aren't actually adding anything to your sentence, but are just taking away 'gain time' that has been earned for good behavior. You can accrue gain time and

They aren't actually adding anything to your sentence, but are just taking away 'gain time' that has been earned for good behavior. You can accrue gain time and eventually earn early release. However, they can also take this away for bad behavior. Obviously some sentences are not eligible to earn gain time.