The arc did not strike the cauldron itself.
It cut in at a tricky angle-right at the point where the cauldron was swelling most violently and its wall was weakest.
The shock wave was "split" at that point.
The explosion's energy swerved sharply upward along the path created by the Deflection Charm, slammed into the ceiling, and blasted out a shower of sparks and rubble.
With a Vanishing Spell from Snape, the debris disappeared in midair.
He put away his wand as though he had never drawn it.
That command to stay still seemed more like an opportunity he had deliberately created for Draco to put on a solo performance.
Then Ron wiped the plaster dust off his face with his sleeve and said in a rather uncertain voice-
"...Malfoy?"
Draco stood where he was.
His wand was still raised.
His hand was shaking.
He glanced down at his hand.
Then he put his wand away.
"Don't look at me like that, Weasley."
His voice was as rough as sandpaper.
"I just didn't want to get splattered."
"Shut up!"
Professor Snape looked around with a sneer.
"Have I not told you that Potions Class does not require you to foolishly draw your wands?"
"However, Mr. Malfoy of Slytherin saved the Gryffindor prefect's cauldron. One point awarded. Five points deducted from Gryffindor."
"The cauldron is already ruined."
His voice returned to its usual icy tone.
"Longbottom, you sliced your valerian root too thickly. The standard was three millimeters, and you cut it five millimeters thick. Your moonstone powder was not finely ground enough, resulting in an uneven dissolution rate. The internal pressure differential exceeded what the cauldron's walls could withstand."
Neville kept his head down.
"I'm sorry, Professor."
"An apology won't save your face."
Snape walked over and cleared away the