MaudlinNamechange
the English way of eating keeps the elbows low and close to the sides. People who scoop and shovel jab you in the ribs and it is annoying at the table.
Also, people who eat with the fork in the right hand tend to do some really gross sort of "processing / sorting / cement-mixing" things with their food. they stir it about and sort of sift and maul it, making it as porridgey as possible. They never just take a bite of something like, e.g., rice salad; there is always a sort of flipping and mixing first, which is really vile
Also: a lot of people who make a song and dance about how manners don't really matter, are designed to oppress with a secret code, it only matters how we treat each other, which should be with consideration, salt of the earth, blah blah - my BIL is a classic one of these and an utter selfish shit at the table. Greedy and selfish. Etiquette can admittedly sometimes be a bit arcane, but the basic principles all derive from making communal experiences pleasant for everyone through being mindful of how one behaves in their presence. People who think "all this shit doesn't matter" are often saying, without knowing, "I am feeling oppressed by having to consider others than myself; offering things; watching to see what people want; behaving in a way that doesn't just put myself first". You might be oppressed, mate, but I am oppressed by you deciding not to bother and scoffing everything in front of you as soon as it is put on the table while the hosts are still on their feet and carving, etc
Stop generalising, or rather, stop assuming that people who eat with a knife in the left hand are the same as your BIL. You are projecting somewhat there.
My DS and I eat with knives in our left hands. Doesn't mean we are automatically selfish and putting ourselves first, as you seem to imply.
What I am saying is that there is no good reason for this particular piece of etiquette. My family are considerate of others and simply choose to differ in which hand to use a knife. And I would guess the same is true of the many other posters on here who eat the same way.
Etiquette and manners are not the same thing.