Flexi Working, or FWS, is essentially a way for the company to maintain the operation, save on sub-chartering and in a small wall keep the headcount pilotwise lower than strictly required for the program.
They fill the shortfall in pilots by getting those on a Legal Days Off to volunteer to work for an agreed fee. It is a win-win for the company and the pilots.
Those that want to work extra can do, and the company saves on any sub-chartering and not having to employ the "extra pilots" they would have needed. Please don't misunderstand the company is not rostering on a shoestring but there are clearly occasions where the is a more significant shortfall in man-power than can be covered by standbys.
All you do is take a look at your roster and pick the days off you are prepared to work, submit them online and then wait. Crewing will call offereing you a duty should there be one. If they ring 3 days in advance of the duty you must accept the duty. But if they ring you within 3days of the duty you can respectfully decline the duty. So in a way you are entering a contract to work the Day off if they call you 3days ahead.
If there are no volunteers then crewing have been know to 'cold-call' people in which case it is purely discretionary as to whether you accept a duty or not.
Obviously night duties that encroch into two days off trigger a double payment. So far as FTL is concerned you must still comply with the 8th day off rule and all the other stuff so occaisionally working a Flexi can disrupt the remaining roster period - but essentially thats crewings problem.
What follows is not strictly in the spirit of FWS but highlights an example of the above:
A captain I flew with the other week worked 2 flexis on his days off during the week which then required him to have the following weekend as days off in lieu as he had now been on for 7days. He ended up getting a weekend off that was originally rostered as night flights. Jammy eh... !