Plug the little motion adapter in to the bottom of your Wii remote and you will have more precise motion control instantly. Nintendo is claiming to have an amazing 1:1 arm to screen ratio.
Since I don't have a Wii, I can't really comment on how much this will improve playability. Doing a little research however showed me that the systems infrared bar loses the pointer often or just locks up all together. So perhaps this nifty little gadget will come in handy.