Your minion sacrifices themselves and you absorb their magic.
I was going to say that this step seems like an assumption, except Eliezer just made Dumbledore say that was the secret to Grindelwald's success, so...
But while his Muggle allies yet made blood sacrifice to sustain him, Grindelwald would not have fallen.
He never said that his Muggle allies were killing themselves; the blood sacrifice mentioned could easily be from those who were killed in the Nazi extermination camps.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
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: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
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: