Because you asked so nicely, I won't give you the old "put the cow on the frig" answer! :lol:
First of all, it's not just for dairy cows, but also for beef as a cow might ingest something metallic and that would not be good for the cow. Eg. broken fence wire, dropped nail/
The magnet goes into the first part of the stomach (the cow has four parts) and it just stays there, to collect any metals that she might eat. Many farms will do it as a routine practice, such as at the time of the first breeding. Some will only insert it, when they think the cow has a problem, but that's often too late, as by then there's probably signs of infection, which may be beyond resolution.
A compass can be placed on the side of the stomach to check if the cow has one inside of her already.
So now, you can all have fun sharing this tidbit brought to you by your Quilting Board Dairy Farmers ... including OKSewGlad, QuiltE and ????
