Okay so this answers what i've been seeing with some moves in MK11 where it says they're high but actually they can get blocked/hit on crouching opponents. I messed around with it a bit myself. For example Noob his F4 is a 2 hitting attack that goes low high but the 2nd attack cannot be crouched, it either hits or it's blocked.
You can actually set the dummy to random block in MK11 to help practice this hitconfirm link, but it isn't as self explanatory in the settings.
AI OPTIONS:
BLOCK MODE: RANDOM ATTACK
BLOCK TYPE: NORMAL
MOVEMENT MODE: DUCK
REVERSAL MODE: OFF
If the d1 hits you seem to have a 2f window to get the 1 out in time where it won't whiff on a crouching opponent.
The timing itself is something you can learn, no doubt, however what the actual issue seems to be is doing d1, visually confirming AND deciding to go for the 12 string. This honestly is very hard because it's 2 fold. You have to be very sharp on your reactions and also have to be very precise in your timing. I'm not going to say this is impractical as i'm sure someone will be able to do it consistently, however this honestly is more effort than it actually should be.
Regardless, thanks for the vid.
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Tested it a bit more, basically any move that leaves you with enough ON HIT/BLOCK frameadvantage you can perform an unduckable high string starter(if it reaches).
I'm no skarlet main but for example she can use f4 in the corner(less pushback) and it allows her to do the 124 string. f+4 is also safe and easy to hitconfirm.
Another example of a blockstring would be f4(hit), 44. 44 is +6 on block.
Thinking even more on it, if you do a j2, where frameadvantage will change depending on how high you hit them, you can do:
j2, 124
j2, 44(+6 oB, launches oH)
All high strings which are unduckable then. Not sure if this is new to you guys, but it is to me lol.