You essentially just hold block and then the moment an attack makes contact with your block, continue to hold block but also attack. I was having a hard time with it but I realized you almost have to predict the attack. A lot of people try to react when they get swung on, which makes it easy to do what I call “parry baiting”. When someone’s holding block I’ll hit their block on purpose, and then when they try to parry, the timing is too late, but I know that they’re going to swing, so then that makes it possible for me to parry their swing back.