I experienced this too, when I started reading out loud more. At first, it was just that my eyes would scan ahead a bit from what I was saying, to help me get the right emphasis by knowing where the sentence was going. It felt like I had "handed off" saying the words out loud to a "subroutine", so my attention could be on what I was reading. Then that "readahead" extended to a whole sentence. And at that point it was like I was so far ahead of what I was saying that I had time to think about it a bit. And then at some point it was like the "reading the words" part got handed off to a "subroutine" too, so my attention could mostly stay on whatever I was thinking