In the Flight of the Prince chapter of book 6, Snape is able to block Harry's spells, including the Cruciatus Curse, which Harry casts several times against Snape. According to canon, the Unforgivables cannot be blocked. I know that one can resist (Imperius Curse) or endure (Cruciatus Curse) but not block them. So, how was Snape able to do so?
Here are excerpts from the scene:
“Cruc —” (Harry)
But Snape parried the curse, knocking Harry backward off his feet before he could complete it; Harry rolled over and scrambled back up again
“Cruc —” yelled Harry for the second time, aiming for the figure ahead illuminated in the dancing firelight, but Snape blocked the spell again. Harry could see him sneering.
“No Unforgivable Curses from you, Potter!” he shouted over the rushing of the flames, Hagrid’s yells, and the wild yelping of the trapped Fang.
I'll preempt two possible answers:
It has nothing to do with the fact that Harry is casting them verbally. That's the only way we see Unforgivable Curses cast in the books. At any rate, that has nothing to do with the ability to block them.
If you are going to say that Snape's able to block it because Harry did not get to complete the incantation, I'll remind you that based on the series, you don't necessarily need to finish an incantation for a spell for it to start working. There are many instances when a caster's verbal spell is interrupted but the effect of the spell is still produced, like:
From Beyond the Veil chapter of book 5
The words were hardly out of his mouth when the female Death Eater shrieked, “Accio Proph —”
Harry was just ready for her. He shouted “Protego!” before she had finished her spell, and though the glass sphere slipped to the tips of his fingers he managed to cling on to it.
A jet of red light had shot from the end of Bellatrix Lestrange’s wand, but Malfoy had deflected it. His spell caused hers to hit the shelf a foot to the left of Harry and several of the glass orbs there shattered.
With all of that, I'll repeat, do we know why Snape is successful in blocking Harry's Unforgivable Curses?