You injure and disarm him.
For example, your blow strikes his wrists or hands; or your sword pommel hits him in the face, or your your blade cuts across his stomach and his natural reaction is to drop his weapon to hold his guts in.
However, you may want to give the player the option of not hurting the opponent if the intention is to purely disarm. RAW then you'd roll damage too, but you don't have to... remember, a disarmed opponent is at a serious disadvantage and it could be good opportunity to secure a ransom or some other advantage from letting your opponent live.