its a deflect , you have to dodge into the attack at the same time it blink , then you hit heavy attack when you hear the sound of the weapon hitting your shield .
As it is a heavy attack you can follow with a shield crush + wathever you like to do after a shield crush
Best place to practice it its duel vs AI as they dont guardbreak too much or advanced training against the capitain . Raider level 1 in training is not the best to practice.
Once you ll trained it well and feel confident. I'm telling you , you will suffer a lot in real fight connecting it ! but its one of the best feeling in the game when you succeed one or you killed your first nobushi by bleeding 

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