In Slytherin traditions were important. The older the tradition, the better. And the Greengrass family always strived to be the best. But at this moment Daphne realized that sometimes there were reasons for change.
Being hit by a Levicorpus during the inquisitional squad raid, showed her that.
Harry didn’t know she followed that tradition. He didn’t even know that particular tradition existed. Had he known, he would have used petrificus totalus instead.
But as he didn’t, him and all the other boys at the scene got a sight that they would remember for a very long time.
Edit: why do people think its a comedy/joke prompt?