Time to hit the lab i guess.... Thanks a lot, I think i finally understand what do now just have to practice.There's no secret really. Like with every other cancelable move, you need to finish input of your special during startup frames of the move you are intending to cancel (SF in this case).
I suppose your best bet would be inputting fb slightly in advance (not too hard when you do WP~SF~WP) then be on point with your f+4 when Liu starts his dd1 motion. I recommend to stagger f+4 from the rest specifically because MKX has a story of losing inputs from buffer when working with 3-motion inputs like dbf, maybe this one isn't an exception too.
If that's correct, you may say that pressing last f and 4 simultaneously is a "secret" you're looking for. To me this was a secret to cancelling strings into grabs
Timing is still rather strict though.