Ok here's a hint:
Master, DestructionBomb, XcaliburbladeZ and Back Turned Legend did it because they already know and use technical characters in a highly technical way, and utilize frames, so if you really want to be able to FSDC do the following (you might think it's lame):
1- go to training -> combo challenge
2- select
and do his last [Juggle (DIFFICULTY 4)] at least 4 times in a row.
or
2- select
and do his [Basic move (DIFFICULTY 4)] at least 4 times in a row.
small hint: change the
to either
or
before the
or
2- select
and do his [Juggle (DIFFICULTY 5)] at least 4 times in a row.
small hint: use
for every choryo
or
2- select
and do her [Combo from a hold (DIFFICULTY 4)] at least 4 times in a row.
3- try doing at least 1 FSDC in a 15-20 minute free training session with info -> move details on 3rd page and the unholdable frame is the one when the numbers change for followup interval as said already.
here's the input to 1 FSDC in a 1 minute practice session I just did
4- if you couldn't then wait until someone does a slow-mo video tutorial or something.
P.S. tell me which character you use (if not one of the four above) and I'll try to help you finding which combo challenge might give you enough training to do an FSDC
