Anti-decay weeks are made to avoid having your account decayed. Basically this means we will play the amount of games needed to be sure that your rating won't drop due to inactivity for the next 7 days.
We usually try to win all the games we play and below GRAND MASTER (4000SR) we have a solid 80%-90% win rate. Most of the times you will find your Skill Rating increased.
That won't happen. And in case it happens, we will play until your account gets back to the original SR for free, without additional costs.