I've heard that chiropractors hold infants upside down by their feet to treat colic.. is this helpful?

No: I suppose the people who put up with this are running out of patience. There is no evidence based studies that support this type of therapy if indeed anyone is using it. I find this description to be disturbing & bordering on abuse. Hip injury and or dislocation is a possible outcome.

Sometimes: When a chiropractor holds an infant upside down it isn't so much for "colic" per se, but to allow the neck to decompress, because some infants have, to a lesser or greater degree, some level of something called torticollis and hanging them upside down can sometimes be just the thing they need to adjust that for them (the infant).

I would not: Recommend holding an infant upside down by their feet. I repeat...I would not recommend holding an infant upside down by their feet! please discuss aany issue you have with your pediatrician.