You can use a Heavy Weapon to cripple a limb or take out a medium- or lightly-armored target.
When you strike an enemy with a Heavy Weapon in a part of their body that is not protected by Heavy Armor, you can spend 1 Valor Point and call out Impale to inflict the Maimed condition on one of their limbs or reduce their Hit Points to 0, based on that call’s description.