It has an effect on the output, but not because of any output buffer as far as I know. It's just my preferred way of encouraging the AI to be more precise.

Another prompt that would work the same way would be: "Ensure your response does not include any of the above words."

ChatGPT (at least 3.5, I cannot say anything about GPT-4) will "understand" better when you reiterate your most important constraints.