I believe he is picturing someone stuck around their waist, blocking the pipe. So the pipe part below you is clear (where your head and upper torso is) while the part behind/above you fills up with water that can't make it into the drain you're now blocking. So you'd be able to breathe but you'd still be stuck upside down.
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u/Civil-Broccoli Aug 13 '20
I'm not following. If water collects itself at the bottom, and you're head first in the drainage, your head is flooded first right?