[Aussie Mike]

That's the D shapes LS6 exhaust port. They flow much better than the round LS1 exhaust port.

You've got to think about how the gasses flow. They stick to the outside of the bend so the floor of the port is a bit of a dead zone. By filling in the floor of the port the port will flow the same for less port volume thereby increasing gas velocity. Or so the theory goes.

Cheers