Wizet's method for using the Patronus Charm was best practiced underwater at the beginning.
Harry understood this training approach deeply. When Lupin had taught him the Patronus Charm, he had used a similar method.
Once everything was ready, Cedric, Draco, and Harry followed the rope into the bookcase.
Only after Draco entered the water did he realize how freezing the water at the bottom of the lake was. No wonder Harry had been shivering so badly.
A bone-piercing chill struck them at once. Draco was not the only one to feel it; Cedric and Harry clenched their teeth tightly as well.
They could not withstand the biting cold. The three of them moved almost in unison, hunching over like boiled prawns and shaking violently as though sieving grain, utterly unable to control themselves.
"Brr... Pa-Patronus..."
Harry and the others tried desperately to cast the spell, but their teeth seemed beyond their control, clacking together whenever they opened their mouths. They could not even recite the full incantation.
Once they had begun to acclimate to the temperature at the lake bottom and could barely manage to speak the spell, they discovered that the surging currents around them also interfered with their ability to complete the Patronus Charm.
After the three of them had remained underwater for a while, a silver glow appeared around Harry's chest, followed by Cedric's and Draco's.
Using the faint warmth from their chests, they hurriedly tugged on the rope and left the lake bottom.
After staying at the bottom of the lake for so long, when they returned to the classroom and pressed against the cold floor, they actually felt a trace of warmth.
"Did... did you... did you see that?" Harry's teeth were still chattering. "That's... that's why I went... went in first! I... I was