″...so shall it be,” Harry repeated, and he knew in that moment that the content of the Vow was no longer something he could decide whether or not to do, it was simply the way in which his body and mind would move. It was not a vow he could break even by sacrificing his life in the process. Like water flowing downhill or a calculator summing numbers, it was just a thing-Harry-Potter-would-do.
Velorien comments on Harry Potter and the Methods of Rationality discussion thread, March 2015, chapter 119