Perfect Parry is the first Deflection technique, a last-moment block that sets up a deadly counterattack. It is the root of the whole Deflection tree and a main source of Resolve in a fight.
Wait for the enemy's swing and tap block as late as you dare. Mistime it and you get a normal parry or block instead of the counter.
Every other Deflection technique sits behind it, and the Resolve it gives feeds healing and Mythic attacks.