"Because once you’ve mastered it, it is extremely easy to cast again, and again, in rapid succession," he explained, "no need to try and think of what curse you want next, and no need to say an incantation. It’s essentially like being slapped with magic - if you cast a strong enough bolt, you can disorientate your opponent and do some real damage," Draco gave a contemplative hmm, and crossed his arms over his chest, "Any other questions?" no one said anything, "Excellent! Get into pairs and we’re going to start practicing expelliarmus," there was a great scrambling of chairs as the group leapt into action, eager to begin.