Grouse, while I aint the sharpest tool in the shed, I aint the dullest. I'll try taking a crack at how and why they work well.

You have forward velocity accompanied by rotational velocity. When the whole system impacts the petals are designed to open, penetrate till a certain amount of force is achieved against the pre- stressed petals, then break off. The rotational velocity achieved from your rifling adds to the velocity of separation of the petals maximizing damage. The solid core then goes on through, breaking bone and whatever else is in it's path and passing through the other side for 2 holes. That core also looks like the edges where the petals break off are a bit on the sharp side as well.

I would imagine this would be a fairly hard system to come up with for various game. Being that all big game are of different hide thickness and bone structure so calculating the precise depth to stress the petals to maximize penetration before separation to cause the most damage would be difficult at best.