Something that could indicate trying to dodge, or consciousness leaving the body. It's not unreasonable for Voldie to think "I've lost here, no matter what I do this body will be unusable in the near future, in case he has a plan to incapacitate me without triggering my Horcrux wards I'd better go someplace else".
It is implied that while Voldemort can stop possessing a victim at will, he cannot stop inhabiting his own body at will.
[EDIT]Though Harry internally states the opposite:
He cannot be imprisoned, for he can abandon his body at any time.
This is the part I was thinking of:
The soul he'd created for himself had to be anchored in this brain, it mustn't be allowed to float free.
Voldemort had the ability fly free from his body. The last sentence you quote is Harry thinking that he mustn't allow that to happen, not that V doesn't have the capability. If V goes free then he must be defeated again; Harry is avoiding that outcome.
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 114, and also, as a special case due to the exceptionally close posting times, chapter 115.
There is 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.)