Well basically, if you have a special occasion coming up and you want a bit more freedom to eat and drink without coming off plan you can have a flexi day. You decide in advance how many syns you allow yourself - say 40 - and you stick to that higher level rather than the usual 5 to 15 syns. Next day you get back on plan and continue as normal. OKay you may not lose that week but the idea is that you have remained in control of your eating by sticking to the target instead of just eating uncontrollably, then feeling guilty and falling off the wagon completely. It's only supposed to be a once in a blue moon thing rather than a regular occurence, and certainly not a weekly event.
I thought it would be worthwhile bumping this just to clear up the confusion on what flexi-synning is. Some people seem to be getting it mixed up with using your syns on a weekly basis when it is something very different.