I interpreted the ease of casting the spell as a specific application of scope insensitivity rather than a change in the requirement to cast it. That is, while casting it the second time might be just as difficult (i.e. take as much mental/magical/spiritual energy) as the first, the third and fourth time would together be only as strenuous as the first, as would the collective fifth through eighth time, etc. It is already established in-universe that some form of personal mana depletion exists, and my idea of this difficulty reduction is an extension of that form of energy to the spiritual energy (established in canon w.r.t. horcruxes, dementors, etc.).
Edit: New thread posted here.
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 86. The previous thread has long 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.
As a reminder, it’s often useful to start your comment by indicating which chapter you are commenting on.
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: