As an addendum to my post above, the MF contract also says thay they could be stood down without basic pay for periods of time. Does that mean that if they reach 900 hours in 9 months they'll be temporarily "laid off" until they can legally fly again?
I very much doubt it, as who's going to stick that for more than a year or so? MF would likely be a shrinking fleet rather than a growing one under those conditions.
No, I suspect that it's there as a "get out of jail free" card for the Company if the airline suffers another 9/11, or similar, when BA faces possible extreme hardship, where it's got the flexibility to quickly cut it's overheads in a time of vastly reduced operation, again without relying on crew voluntarily taking unpaid leave, part time, VR etc.
Essentially, it's just a very flexible contract (for the Company), many parts of which, it hopes never to have to use.