Physical attacks trade Def for Spe.
Weak Armor is an ability that causes the Pokémon to lose one stage of Defense and gain two stages of Speed when hit by a physical move. This means that when a Pokémon with Weak Armor is struck by a physical attack, its Defense stat will decrease, making it more vulnerable to subsequent physical attacks, but its Speed stat will increase, allowing it to outspeed many opponents in future turns. This ability can be strategically used to turn the tide of battle by sacrificing some defense for increased speed, enabling the Pokémon to strike first in subsequent turns.