This first clip shows a standard Flatten (FL050, for boats from approx. 30 to 55ft) rolls. The key point here is that as the leading edge splays open (as it does when it comes toward the headfoil) the Flatten naturally wants to roll up.
Once rolled, you can see that the Flatten is stable. This means that the furled sail is held tighter than it might be with no battens at all, and certainly tighter than it might be with a furling batten that wants to straighten out.
With the Flattens in their open state, they are stiff in both directions. The main difference to the Flattens and a traditional batten is that they if they bend more than a certain %, they simply fold up.
For this reason, the Flattens are only used in the flatter sections of the sails. In this clip you can see the bend characteristics of the FL030 Flatten (for yachts of approx. 20 to 35ft).
And like any non-furling batten, if the Flattens are rolled the "wrong way" or if you "fold rather than roll" by hand, then the performance could reduce. In the next two clips they show what might happen if the Flatten is folded at the leading edge (rather than "splayed" as in the previous video) or, in the very unlikely situation, that the Flatten is rolled the "wrong way" the Flatten is obviously not perfect after, but can still be used in the sail if no others are available.
