I was paraphrasing from my mother tongue, so idk if the term is 100% accurate. But in a nutshell, many older houses (at least where I come from, in Europe) don’t have machines that take care of the airflow/changing fresh air, but rather have systems designed around the natural airflow based on gravity. In these kind of homes, making DIY adjustments might break this gravitational airflow, causing mold issues and bad air quality in general.

I don’t know if this is a thing in 'murica but I thought my small addition might save someone a house, hence I decided to include it in the conversation :)

A word of advice: if your household has a gravity-centered airflow, please do NOT block all small gaps, else you might develop nasty air quality-related problems later on