If it doesn't work in UMK3 then its neat but worthless outside of MKT.
Sounds like he is talking about a MKT specific priority thing with the pushback.
From his own subs:
"This combo will not work in the 1P side," 0:57
"with the same input I have done." 0:58
"Will not work with perfection," 1:01
"She will be pushed back after a few hits" 1:03
"See? As much as I try, this will not work" 1:04
"So, Sindel on the 1P side is 'blocked'," 1:07
"and the 2P side Sindel is 'unlocked'" 1:11
"For this combo you need two Sindels," 1:17
"to learn this combo" 1:20
"In the course of time you will known when the round is 'locked' or 'unlocked'." 1:22
"If by the way you try do the combo," 1:27
"in the 'blocked' round," 1:29
"when she is pushed back in those first flights," 1:33
"forget about it, will not work." 1:36
"Now if you try in the 'unlocked' round," 1:39
"the combo becomes incredibly easy." 1:43