Dumbledore uses a wordless body-bind curse on Harry to stop him from interfering in his confrontation with Draco

Extract from Harry Potter and the Half-Blood Prince

by J.K. Rowling

Harry hurried over to the door leading to the spiral staircase, but his hand had only just closed upon the iron ring of the door when he heard running footsteps on the other side. He looked round at Dumbledore who suggested to him to retreat. Harry backed away, drawing his wand as he did so.

The door burst open and somebody erupted through it and shouted: 'Expelliarmus!'

Harry's body became instantly rigid and immobile, and he felt himself fall back against the tower wall, propped like an unsteady statue, unable to move or speak. He could not understand how it had happened - Expelliarmus was not a Freezing Charm -

Then, by the light of the Mark, he saw Dumbledore's wand flying in an arc over the edge of the ramparts and understood ... Dumbledore had wordlessly immobilised Harry, and the second he had taken to perform the spell had cost him the chance of defending himself.

Standing against the ramparts, very white in the face, Dumbledore still showed no sign of panic or distress. He merely looked across at his disarmer and said, 'Good evening, Draco.'

Malfoy stepped forwards, glancing around quickly to check that he and Dumbledore were alone. His eyes fell upon the second broom.

'Who else is here?'

'A question I might ask you. Or are you acting alone?'

Harry saw Malfoy's pale eyes shift back to Dumbledore the greenest glare of the Mark.

'No,' he said. 'I've got back-up. There are Death Eaters here in your school tonight.'

'Well, well,' said Dumbledore, as though Malfoy was showing him an ambitious homework project. 'Very good indeed. You found a way to let them in, did you?'

'Yeah,' said Malfoy, who was panting. 'Right under your nose and you never realised!'

'Ingenious,' said Dumbledore. 'Yet ... forgive me ... where are they now? You seem unsupported.'

'They met some of your guard. They're having a fight down below. They won't be long ... I came on ahead. I - I've got a job to do.'

'Well, then, you must get on and do it, my dear boy,' said Dumbledore softly.

There was silence. Harry stood imprisoned within his own invisible, paralysed body, staring at the two of them, his ears straining to hear sounds of the Death Eaters' distant fight, and in front of him, Draco Malfoy did nothing but stare at Albus Dumbledore who, incredibly, smiled.

'Draco, Draco, you are not a killer.'

Harry Potter and the Half-Blood Prince

by J.K. Rowling