This sounds like a cooking tip rather than a tear-prevention trick, but it makes chemical sense. A blunt knife tends to crush and rupture more onion cells than a sharp blade. Since the tear-inducing chemistry begins when the onion’s tissues are damaged, reducing unnecessary cell destruction can help limit the amount of lachrymatory compounds released into the air.
Use a genuinely sharp knife and make clean, decisive cuts rather than repeatedly sawing through the onion. It is also safer cooking practice, since a dull blade is more likely to slip.