Disarming is an unarmed ability. It allows you to knock the item out of your opponent's hand, by using your fist. your chance to disarm goes up around 1.5% every 50 levels. Do /unarmed to see this. If you are disarmed it will say "You have been disarmed!" In red text. If it says "Your opponent has an iron grip!" That means you were going to disarm them, but their iron grip ability kicked in. A tip for disarming is to disarm them while they are running. Their sword will stay behind them as they run, and you can collect it. Make sure if you see people using their fists on a server with disarming enabled, you take out your fists too, otherwise you have a chance to loose your current hand item, and you don't want that!
A tip from me to you is, if they are a really good guy try to disarm them grab their weapon and kill them. It will be easier