Summoning Snape på toppen av Astronomitornet

Ditt antagande att han inte förlorade sin kraft (som i energinivå, inte förmåga) verkar motsägas av boken: han var upphetsad och allvarligt underpowered:

'I've been better,' said Dumbledore weakly, though the corners of his mouth twitched. 'That potion... was no health drink...'
And to Harry's horror, Dumbledore sank on to the ground.
'Sir – it's okay, sir, you're going to be all right, don't worry –'
He looked around desperately for help, but there was nobody to be seen and all he could think was that he must somehow get Dumbledore quickly to the hospital wing.
'We need to get you up to the school, sir... Madam Pomfrey...'
'No,' said Dumbledore. 'It is... Professor Snape whom I need... but I do not think... I can't walk very far just yet...'

Notera här att Dumbledore knappt lyckas tala, pausar. Det är ganska klart att han skulle ha svårt att göra vad som anses vara en svår och avancerad stavning (Expecto Patronum) i detta tillstånd; när han knappt kunde stå eller prata.

'Right – sir, listen – I'm going to knock on a door, find a place you can stay – then I can run and get Madam –'
'Severus,' said Dumbledore clearly. 'I need Severus...'
'All right then, Snape – but I'm going to have to leave you for a moment so I can –'

Se, att mörkmarket fick honom att nå sina reservationer, vilket innebär att han lyckades med den sista gaspmagiken som slutsäkerhet och kroppsbindande Harry:

'We need to return to the castle at once,' said Dumbledore. 'Rosmerta,' and though he staggered a little, he seemed wholly in command of the situation, 'we need transport – brooms –'

'Rosmerta, please send a message to the Ministry,' said Dumbledore, as he mounted the broom nearest him. 'It might be that nobody within Hogwarts has yet realised anything is wrong... Harry, put on your Invisibility Cloak.'

En tredje anledning - för att skicka Harry medan han var på astronomitornet - var för att skydda Harry . Dumbledore visste att Death Eaters skulle komma för honom på grund av Malfoys arbete:

'Go and wake Severus,' said Dumbledore faintly but clearly. 'Tell him what has happened and bring him to me. Do nothing else, speak to nobody else and do not remove your Cloak. I shall wait here.'
'But –'
'You swore to obey me, Harry – go!'
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 gestured 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.