The Kanohi Hau was the Great Mask of Shielding. The mask created a force field around the user, protecting him or her from physical attack. Its sole limitation was that it could only protect against attacks that were expected, so it was ineffective against ambushes.
The Hau was the Great Mask of Shielding. It provided protection against any attack the user was aware of, but not against ambush.
The Kanohi Hau was the Great Mask of Shielding. It protected its user against attacks but had no effect against ambushes.