Mike,
You are correct, the carbide "button", is pulled through and swages the rifling into the bore. This is a forming process not a cutting process as broaching would imply. The process is usually very consistant, with a very fine bore finish, and produces a better than average accuracy, with less cost than cut rifling.
The hammer forged barrel cost a lot more for up front investment in machinery to do the process. Hammer forging is also known as the "Appel process" or cold forming.
Button rifling starts with a bore that is under sized and is swaged by the button, and hammer forge starts with a bore that is over sized, and is rotary hammered around a mandril, "plastisizing" the steel and forming to the reversed rifle image on the mandril. The advantage of this is molecular rearrangement that gives a harder surface. Again if the mandril is used to long or the surface is damaged, no matter how little, this is imparted to the rifling in the finshed bore.
MaBell