Short Answer:
cr.M, s.S cancelled (xx) into a Tiger Knee’d (TK) special move (NS = Neck Slicer and the ‘MH’ was supposed to be a MF** for Mirage Feint).
And the answer to the short answer (just a heads up, this is gonna get really nerdy, so I’ll try to explain it as best I can. Sorry if you already know all this. I’m sure someone can get some use out of it and I can’t get to sleep anyways.):
First part is explaining the number notations (just in case anyone hasn’t seen it already), 2nd part is explaining Tiger Knee’ing, 3rd part explains launcher cancelling, and the 4th part is putting it all together to answer your question (this turned out to be a long freaking post.)
Number notations yet for directional inputs:
If you see someone write 214 L, they’re saying do a Mirage Feint L (2 = down, 1 = down back, 4 = back, this makes sense if you have a NUMPAD on your keyboard).
623 L would be a dragon punch L. (6 = forward, 2 = down , 3 = down forward)
2369 L would be a Tiger Knee’d Neck Slicer or Tiger Knee’d Talon Attack L depending on how fast you press the L.
Normally, Tiger Knee’ing is when you do a special move but end in a up-forward (9) or up-back (7) direction so that you can do an air special move as low to the ground as possible. This only works because the game has some leniency in reading the inputs. When you put in 2369 L, it read the 236 as a quarter circle forward and stores that, then reads the 9 as a jump, but once it gets the L input, it will give you a very low talon attack instead of a j.L. The reason this is useful is that it’s much faster to do 2369 L than it would be to do 9 to jump and then a 236 L just because of the physical motion. (TK TA L is really great for pressure since there’s so much hitstun on the Talon Attack and it’s much harder to react to close to the ground.)
So now, Launcher Canceling:
Normally, You can only superjump after a lauch but with how the game handles jumps (the first 3 frames of a ‘jump’ you’re actually still on the ground) if you tiger knee a motion fast enough the game will register the up-forward as a jump, but then cancel those 3 pre-jump frames into a ground special move (if you’ve messed around with Dante any, this is how his Bold Cancel options work). Sometimes you’ll see this called launcher cancelling if you’re using this to do something after an S. I don’t know if there’s an official term for it, though, since it’s not used that often. This only works on hit since you can’t super jump out of an S on block or whiff.
So for those random S xx TK Neck Slicer ammy combos:
s.LL+Ammy, s.M, cr.M s.S xx TK Charged NS (2369 L or down, down-forward, forward, up-forward L) will let you ‘launch’ them with an S while Cold Star keeps them near the ground, then hit them with a charged NS. You can just as easily use a s.HxxCharged NS, though. I was just being dumb, Lol.
You can see the difference if you do the s.LL+ammy, s.M cr. M S xx TK MF H (2147 H) j.MS and it will spike them to the ground into a hard knockdown even through you’re really low to the ground. If you just used cr.M s.H xx MF H j.MS they would be left in the air like normal.
You’ll very rarely use this technique as X-23, but there’s a few assists like Tasky’s vertical arrows or Striders Vajra that she can use that to extend a bit. Launcher cancelling with assists like that lets you abuse the fact that S launchers have a ton of hitstun on them and do stuff like S+vertical arrow (arrows hit them at the top of the launch and knock them back down to X23)xx TK Charged NS to get a spinning state into another launch. Dormammu uses this all over the place in his advanced combos for stalking flare juggles since it lets him teleport up the them and get a hard knockdown into a 2nd super.
Seperate from ALLLLL this nonsense, I would recommend learning how to Tiger Knee X23’s Crescent Scythe (62369 H) attacks since the game has a wierd thing where it will let you cancel into a Talon Attack from a TK CS faster than if you did a regular CS. If you do a TK CS H you can actually cancel at any point rather than having to wait for the full animation.