These types of mills use a series of rotating metal fins that move very fast. When the wheat passes through them, the fast impact breaks up the wheat into smaller pieces.
This type of milling is actually the original way wheat used to be ground into flour. Traditionally, the burrs were made out of stone, but in modern times, these burrs can be made from stone, cast iron or steel.
If you’re going to be needing to make huge amounts of flour, you should consider buying either a grain mill that is going to mill fast, or a mill that is going to carry a large amount of grain.
If you want to have the option of a super fine flour, then a burr grinding mill is a good choice. An impact mill, although will get you fine flour, won’t produce flour that is super fine.