Interesting. I've always seen people actually grabbing the sandpaper when sanding, not grabbing the object to sand. You have way more experience on that than me, so I'll just step aside on this one
Usually you would do taht this way, but it's not the case with CPUs or heatsinks. If you held the block wrapped in paper in hand, you wouldn't get flat surface - it would be curved, probably higher at the middle and lower at edges. Hand is just not that stable and steady, unless you're a Terminator.
And polisher is too fine to remove that nickel plating form IHS, it was used only to make a better mirror I'm not sure if using polisher is good for getting really flat surface.