I'm analysing the trailer...
So Mitsu assumes a stance before he is hit, and his sword glows red.
Soph hits him with 4AAA and they're all deflected including the low.
Mitsu swings, hitting Soph before she recovers.
But, she seems to enter a guard broken state, and the characters are sent backwards quite some distance. (Perhaps they edited to a scene where she actually was able to block it, and this is the result of doing so, otherwise, very strange.)
After that, we cut to some clashes. Not sure if they're part of the mechanic or not.
Then we have Soph dodging the same red slash from Mitsu. It looks like a stance of some kind to me, like Raph's.
Then we have Mitsu attacking Soph while she deflects...
She hits him before he can recover, but again he seems to get guard broken, and both are sent backwards.
Both of the red attacks are vertical, so I'd assume the counter is to just not attack, step to the side, let them swing and punish afterwards. Or grab, or probably super.