By forfeiting your attack you may instead choose to attack their weapon. Unlike attacking a weapon of someone that's holding you at bay to move it out of the way, this is a deliberate strike to deal damage. The attack is against the defender's shieldless defense +6.
On a hit the attacker rolls a reduced damage die that's as follows:
- All piercing weapons only deal 1 point of damage
- All crushing weapons use the lower of their two damage dice
- All non-polearm hacking weapons deal the higher of their two damage dice
- All hacking pole arms use the lower of the two dice rolled and they drop the +3
If a weapon falls it's breakage save, it doesn't shatter but instead the strike lowers the quality of the weapon, depending on the amount of weapon failed by (as per the usual rules found on page 197 of the PHB). After -4 the weapon shatters and is useless.