Is your common sense proportional to the size of your fist?
A "fist rule" doesn't involve any punches or hits and is usually safer than it might sound. It can be practical: 16 degrees Celcius is roughly equivalent to 61 degrees Fahrenheit, for example. Or it can be a half-truth that's open to interpretation (vertical strips make you look thinner) or contains unsound scientific assumptions (brown eggs come from brown chickens).
While in English we refer to such broad principles, however true or untrue they are, as rules of thumb, Germans go for the whole hand and call them "Faustregeln" (fist rules). While they usually don't involve physical violence, they're only dangerous in some cases if not taken with a grain of salt.