That’s not what he said. And that part is from the GTD principle, much older than the “second brain” wave. That GTD principle, backed by psychological research, is basically saying that you should extract what you have in your mind immediately when you have it / remember it, so that you don’t need to keep it in your mind all the time. That free your mind to perform at optimal capacity. In psychological lab test, they basically test people doing “remember X and perform this next task Y. I’m going to ask you about X after you finish Y.” The mere fact that you need to remember X degrades your performance.