Ouch, pricey things, aren't they? I have/had a set of a dozen, from HF possibly, that I used for a long time, but last time I looked, they apparently followed my younger son home. Never saw a need to flatten the backs, did all my sharpening on a belt sander, and they performed great. I've got my eye on a replacement set on ebay, 12 for around $12. Look just like my old ones, made to be whacked with a carving mallet. I don't know, but if I wanted to flatten the back of some, I believe I would start thinking belt sander, possibly disc. Chisels that pricey tho, I'd get some el cheapo chisels to practice with first, maybe yard sale chisels. I don't watch youtube about things like that, so just what are you supposed to be gaining by flattening the backs of your chisels?
"It ain't what you're told, it's what you know." - Granny Weatherwax
Some days, the supply of available curse words is insufficient to meet my demands.
Call me a craftsman, artisan, or artistic, and I will accept that. Call me an artist and you will likely get a quite rude comment in return. I am not a @#$%ing artist.