After Snape has entirely embarrassed Lockhart, he asks for some student volunteers to try out a friendly duel. Unfortunately for you, his finger points directly at your chest and before you know it you are hauled onto the platform and your wand is pushed into your hand. Suppose we're doing this then!
Even more unfortunate is the fact that Malfoy is selected as your opponent, courtesy of Snape, of course! You can see Malfoy sneering at you, his lips starting to move in a whisper that, no doubt, contains some nasty curse. You know he would love nothing more but to make a fool of you in front of all of the students here.
Your heads quickly runs through a somewhat limited catalogue of spells you can apply here (Lockhart really didn't prepare you for this at all!)
Unless you want to get hit by a curse, you have to act now!