The Mask of Kanohi is an extremely powerful Kanohi. It contains all the mask powers that a Toa, anywhere, has used. The mask senses Kanohi powers used across the world and when a new mask power is used by a Toa, it collects the power into the mask. Toa of the Toa was careful with the mask, since it had so much power. That's whyhe rarely used it. Because Reeha cannot leave his position, and because he is good, his is of little danger to anyone.