@Fire Fist:
Guys, it's really simple how Law's power works.
1. He creates a closed space with the Room command (focusing his ability)
2. While in that space, anything he does on his end of the space also happens on the exact opposite end of the space. (i.e. swinging his sword and the Marine's head coming off)
3. When he uses the Shambles command, everything in the closed space switches places. In the instance we saw, he switched the moving cannonball with the marine's head, which were in exact opposite positions from each other. Because the cannonball was moving, the head took on its physics and vice versa, which ended with the head flying toward him and Law catching it.
That's all there is to it. It's a closed space that he can reverse and manipulate as he sees fit.
I agree with what you're saying, but we don't know if that's really the full extent of his power, for all we know right now that's just one trick up his sleeve. For all we know "Room-Shambles" is just one move in his arsenal of abilities and even that we don't know all about from what we've seen, there could always be more to it.
Also, that doesn't necessarily explain why the marine's head was still alive and able to talk as it seems to be out of his Room technique when it's speaking.