All ‘engine turning’ means is ‘produced by machine’.
Engine turning can produce many forms of pattern, from the coarse lines which might be used on a rib to that pattern of tiny, shiny circles you sometimes find on bright parts ? often on rifle bolts or on the barrel cheeks and lumps of O/Us.
The coarse pattern found on a blacked rib is to prevent sun glare, while the tiny circle pattern is more than cosmetic ? it retains a very small amount of oil.