i'm aware i just can't perform it on any of the characters this guy is doing it on. i hypothesize he has some special DI or something. idk how he's doing it. try it yourself it wont work. and i think he's doing it in 1.1.5 but i could be wrong. tweets are recent.
Just DJ backwards into Disable. Even though you cancel the DJ immediately, Mewtwo still retains a small amount of momentum.
It can be done on about 90% of the cast, there's a few characters which are just impossible to get it on because Mewtwo always falls behind them.
As I had told Sonic, FS -> Disable is guaranteed but getting a grounded footstool on a non-shielding, non-attacking opponent without a setup is even harder than just landing a raw Disable. If we had a reliable setup it would be useful but as it stands right now it isn't exactly gamebreaking.
Remember that N-Air makes the opponent face away from you so I believe (although I have not tested) that in order to get N-Air -> FS -> Disable you are required to fall into your opponent and activate N-Air right as you touch the ground.
This is incredibly risky as you are basically landing right in front of your opponent with no hitbox too. I might be wrong and there might be other ways to get N-Air -> FS, I will try and lab it further this weekend, but from a theoretical stand up it seems to work sort of like Marth/Lucina/Roy's N-Air 1 only with a much more strict timing and point blank range (instead of a full sword range).
Keep in mind FS -> Disable has been known for a long time, and falling F-Air -> FS only works on sub 20% if the opponent is DIng up and away and at those %s you can get D-Tilt without FS anyways (although, to be honest I haven't retested this since patch 1.1.1, I mean we went from 1.7 to 2.05 run speed, maybe this can actually be a setup until a higher % now).
Basically these vines are very creative but aside from D-Tilt -> FS which I am unsure if it's guaranteed with good DI, there is nothing really groundbreaking. It's just really flashy.