Extra Easy Days flexy days

I hear about these flexy days but because i dont go to the classes i was just wondering how they work...... you save all your syns up and use them on one day or you eat all your syns each day and then have a day off where you eat whatever you want??? And still loose??

ahhhhhhhhhhhh thankyou for clearing that up for me........ i didnt realise you could save them up and have them for say a chinese.... that would be a good idea