The Hau Nuva was a more powerful version of the Kanohi Hau. It was able to project a stronger force field than the Hau, and the effect could be maintained for a longer duration. Like all Kanohi Nuva, the user could share its effect with those in close proximity.
The Great Mask of Shielding. It provided the user and those around him protection against the most powerful attacks, but not against strikes from ambush.
The Kanohi Hau Nuva was the Great Mask of Shielding. It protected against the most powerful attacks, but not against ambushes.
When Tahu Nuva used the mask, a translucent red force field shot out from the mask, surrounding him in a glowing sphere of energy. He was able to use it to protect Takua and himself from falling lava.
The Kanohi Hau Nuva was the Mask of Shielding. Its power would protect a Toa from any physical attack, provided it was not unexpected. It had the power to protect those near the wearer as well.
If an attacker struck the shield with enough force, it would send the user of the Hau Nuva flying back. The Rahi Nui was able to do this by charging into the shield.
A Hau Nuva user could protect a user against an indirect attack against them that struck their nearby environment because the user wouldn't have any way of knowing the attack wasn't aimed at them. If the attack on the environment was far away (such as dropping a rock on the Hau Nuva user from high above them), the user could not block the environmental attack, but perhaps could block the consequences of the attack (the user could shield against the rock before it hit them).