A more boring way to say this is that "text samples in the Internet that use 'stressful' language are more coherent". Telling GPT4 to act as a "stressed historian" simply selects (or prefers) text samples that appear in the proximity of stressed historians. If there are no such samples, it will generate gibberish, but if there are indeed stressed historians who have produced enough high-quality work, GPT's output will be great.