I believe flexi syns is when you know your going out and you plan how many syns you will need. It's syns outside of your normal 15 day allowance. Say your off out for a meal and drinks, you plan out 30 syns for the meal then say 20 for drinks and stick to it.
Then the rest of the week you carry on like normal having 15 syns a day. It's a planned 'off plan' meal if that makes more sense? Hope this helps x