The Kanohi Elda was the Kanohi Mask of Detection.
The Kanohi Elda was the Mask of Detection. It allowed its user to find the Mask of Life wherever it might be. Hahli was possibly the most important of the Toa Inika because her mask would lead them to the mask they sought.
It allowed Toa Inika Hahli to detect the presence of the Kanohi Ignika and see Matoro's spirit when it was free of his body.
The Kanohi Elda, the Mask of Detection, could find the Mask of Life wherever it might be hidden.
The Mask of Detection primarily detected the Mask of Life, but could also spot things that were invisible.
An Elda user would experience headaches when near the Mask of Life.
The Elda's headache-based detection was exclusive to detecting the Mask of Life.
The headaches were the Elda's way of communicating with its wearer. It would perhaps be able to signal a faster path towards the Ignika even if the path itself was further away from the mask than the wearer was at that time.
If an Elda user was close to the Ignika, it wouldn't create a painful headache unless the wearer didn't notice the Ignika.
The sole ability of Kanohi Elda was to track down the Ignika, which is why there weren't very many Great Elda.
The Mask of Detection allowed its user to see things that were hidden.
The Elda could not detect stress points.
The Elda allowed its users to see things like spirit forms. It also guided users toward the Mask of Life through severe headaches. It did not allow users to see through walls like the Akaku; that was not how it allowed users to sense hidden things.
An Iden user's projection would appear to be a ghost to an Elda user.
Elda didn't help users find the spirits of Iden users, it just allowed them to see the spirits.
An Elda user couldn't detect if a body was posessed by another being's spirit.
The Elda was likely made using disks other than those found in Metru Nui.