Flexible synning is different to using your syns weekly.
Using your syns weekly = 15 syns x 7 = 105 for the week. Use them during the week taking the amount used off the total.
Flexible synning = only used for a special occasion e.g. birthday/wedding/special night out. You decide how many syns you will have that day, you might decide on 50/60 and you count until you get to your total, then stop. It gives you some control so that you don't go off plan completely. You still have your normal daily/weekly syns. You might not lose weight in a week that you have a flexi-syn day.