You decide on an amount of syns you need for a special event (eg 40 syns), and then countdown to zero for that event. this is in addition to your 15 a day. You accept you may not loose that week as a result. It is only for use for special occassions not every weekend.
I've never used it myself. I save syns instead for the weekend, and frankly, if its a big event. I prefer to just go off plan for that one meal/event.