So I went back and watched the Leo vs False set at GOML, to see Leo's usage of FF fair. When Sheik was at 20%, Leo did d tilt, which tripped, into DA, up air, FF fair 1 and 2. He then shielded False's falling aerial and grabbed, continuing the string with a d throw into an up air chain, finishing with an offstage nair that almost KO'ed. In total that was 45%.
FF fair is a scary mix-up, and is potentially as devastating as FF bair locks. Up air -> FF bair into d tilt locks is not true and purely a mix-up, and long ago it became common-place for MK's to incorporate it into their punish game, especially back when it led into early KO's. I think the next step in improving his punish game--or rather revitalizing it--is through exploring what can be done with forward air.
On light fast-fallers at 0%, you can d throw, if they DI away -> tipper DA -> FH up air -> FF fair 1 and 2 -> read their option and grab into d throw -> up air -> up b, for a total of 43%, for example. While Tornado combos are simpler for a high 29%, someone who knows the MU will DI away/down-away to avoid the Tornado, while in those cases DA will cover that DI angle, and for possibly ~10% more damage.
The opponent can air dodge up air -> fair at that percent, but you can still FF and continue the string, though it's dependent on whether or not they suffer air dodge landing lag. You may want to shield after the fair landing lag in case they opt to land with an aerial, and then grab. MK can d throw -> FF fair 1 and 2 as a true set-up, but he cannot land 1 and 2 without getting the last hit if they DI away on d throw.
Such set-ups def aren't reliable but they are useful to keep in mind as mix-ups, especially with how they're still relatively new and underutilized.
