It's a small muscle, looks a lot like pork tenderloin in shape. It's a good eat, cooks rare nicely, but has way more flavour and texture than beef tenderloin. It's somewhat denser, so pickups are a little slower, but not much.
One of the many "last straws" that made me bail on the steakhouse was the owner refused to let me add it to "my" new menu. I kind hate that it's catching on, prices are already way up from this time last year.