I'm pretty sure that is advice from the last millennium that is no longer taught.
Specifically, the two reasons that it's no longer taught is that 1) rushing to get under a doorframe caused accidents 2) doorframes are no longer reinforced the way they used to be.
alt Hacker News
Specifically, the two reasons that it's no longer taught is that 1) rushing to get under a doorframe caused accidents 2) doorframes are no longer reinforced the way they used to be.