This perk increases your chance to hit an opponent's legs in V.A.T.S. by 25%, thus making it easier to hobble the target (reducing their movement speed by crippling their legs). As with most perks that increase your chance to hit in V.A.T.S., this bonus is multiplicative, not additive.
Behind the scenesEdit
In the 1990 movie Misery, a serial killer hobbles her favorite novelist by smashing his foot with a sledgehammer. When asked if the perk references this, Joshua Sawyer posted "Not directly, but the icon does suggest that."