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?