Lock is only applicable to off-hand weapons weilded together with some other weapon - which can be an indentical copy of itself. Use your main weapon to lock your opponents blade, opening him up for an off-hand attack. You can now use the Lock weapon to attack your opponent after his weapon has been caught in a clash of steel.
When you miss an attack against an opponent with your main weapon, you may immediately expend an additional shot and make a normal attack with this weapon.