HOLY ROLLER

[Riotgear]

I've heard two stories - Not sure which is true -

The little post thingy in the center of the bullet is supposed to direct hydraulic pressure against the inside of the HP cavity insuring maximum expansion.

Or

The little post thingy helps the bullet continue to penetrate after expansion partially negating the trade off between energy dump and penetration.

I think the former is more likely than the latter. But I'm no scientist.