The Perfect Guard is a special move used by Link in Breath of the Wild. If Link blocks an attack with perfect timing, it may reflect an attack, or stun the enemy which attacked. This move is good against Guardians, as their beam can be reflected back at them, causing great damage. It can also negate the momentum of explosions from Remote Bombs, Bomb Arrows and explosive Lynel attacks.
Link must have his shield up, and once the attack hits, while holding the shield, the player must press the (A) button to produce the deflection.
The Perfect Guard can be achieved by using all shields and shield-like items like Pot Lids, at no cost to the shield's durability. Link can produce the deflection if pointing at the general direction of his attacker (so he doesn't have to be locked on them).
The Ancient Shield, which can be purchased at the Akkala Ancient Tech Lab, is the only shield in the game for which the Perfect Guard is not necessary when fighting against Guardians, as it reflects their beams on its own, but at the cost of the shield's durability.
The Perfect Guard, in spite of being a special move, does not deplete the Stamina Wheel.
Prompt for the Perfect Guard tutorial at the Ta'loh Naeg Shrine