the parry window in ACU is huge. usually very very easy to get a parry. you can try and time it well for a perfect parry that opens the enemy up for a counter, but that window within the parry window is also big. bigger than the sloppy parry window in the beginning of the attack animation.
the way i always wanted a melee system to be is such that the enemies usually attack fast enough to make it look realistic (no as slow as AC and faster than shadow of mordor though i think the uruks speed in that game is pretty fine) but since im aware that a human reaction makes it really hard to ALWAYS time block when an enemy strikes (in a game. in reality its different and easier to see what you opponent is doing since he is so big within your FOV) so there is a passive guard stance that prevents damage but also staggers the defender, so the attacker can keep on attacking him until the defender loses his balance and becomes open for attacks.
but if you DO manage to time your guard than its a parry that opens the enemy for a counter.
i hope it's sorta like that. from the footage we've seen it appears that it's "almost" like this.
it does seem that the parries that push enemies back are always done when the guard stance was initiated really close to the moment of impact, so one can hope...
also i got a reason to believe that if you hit an enemy who is guarding too much than you recoil backwards. in the IGN video it looks a bit like the player always jumps backwards after the enemy parries him. so i wonder if that is a mechanic...
@thisismadness - i didnt see a video where geralt is guarding yet is being hit from the back. can you point me to the exaclt moment in the video where this is shown?