I actually watched that, but it didn't really help me understand the timing any. I was hoping someone with experience playing her could explain it (maybe if I understood Japanese, the video explained it too, but I don't :x)
If I just spam the input just outside attack range while the CPU is set to attack over and over again, it will eventually trigger, but I can't figure out how to do it on purpose... and I've never gotten it to work when close enough to be hit. I either get hit or on rare occasion do her stand A and trade hits (or interrupt the attack). It never seems to trigger the counter when actually being in range to attack. Obviously I understand nothing of the timing.
you have to input the direction within few frames from pressing A
I have found out using

+

is the hardest since the counter will happen only during the active frame of the medium attack (range doesnt matter)
you can practice by recording Yamazaki

+

multiple times and you will get the timing
usually I go for

+

(just a matter of guessing) &

+

(easy to catch jumping attacks)
I use it against all Choi jumping moves (except Max 2), Namless jump

or jump

and all cross jumps attempts when grounded but be careful as sometimes the counter miss the opponent
I wanted to share one more tip since very long time
instead of inputting

after

or
simply press

+

for the follow up attack
