They need to cast the charm on their hand?!

O________O

He watched older students came up the front of the class to demonstrate the spell. He watched the demonstration very carefully hoping not to miss anything, he didn't want something wrong happen to him.

When the Professor said that now they're allowed to perform the spell. Messer picked up his wand and pointed it toward his other arm. Twist and tap. "Prehenso!" but nothing happen. Maybe he said it wrong. Twist and tap "Prehenso!" then he feel a tingling sensation on his left hand.