Remington has revealed how their new Xelerator Wad technology, which is used in the HyperSteel ammunition, works. An extra powerful primer ignites a column of power contained inside the wad. For very brief period of time wad rockets forward, much like a simple firecracker rocket, before the shells main powder charge takes over.
I suspect that the powder enclosed by the wad would be a different blend to the main charge.
[ Many thanks to Matt for sending me the info. ]