Or maybe it's not a good idea to cast incendio while the tip of your wand is in a very enclosed location (for instance due to gases that you expect to be released).
Or it's maybe that that the transfiguration requires no wand movements (reference: look at the setup used during the partial transfiguration experiment) as opposed to incendio. It's pretty hard to move your wand in some pattern when it's stuck through a troll's eye socket.
It's not a fire vulnerability. It's just that chemical change stops the regen.
Just because burning cut troll flesh makes it not regen doesn't mean that it will penetrate. They're not highly flammable like Twilight/Luminosity vampires. If I was designing Muggle anti-troll weaponry, I'd be going for something to penetrate into the body and then spread incendiary material. Not a flamethrower. (Although first physically injuring trolls and then dousing them with napalm might work, sort of.
Incendio in the troll's eye would just burn out the eye.
Mana cost in ...
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 88-89. The previous thread has passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: