For understanding why the hair becomes such an important element, we have to get into its symbolic meanings. According to what we have read in religious texts, mourning, hair and resurrection are the three pillars of the believing.
The mourner gives the hair sema while she cries, weeps and regrets the death. The weeping and the mourning happen when there is disorder. In the Osiris legend, when the god died, the world, with no governor, was in a big chaos; the death of Osiris meant confusion, darkness and disaster.
In this context we could think that the nwn gesture of shaking the hair and covering the face with it would symbolize the chaos and darkness produced by the death; mourners hide their faces and cannot see in the same way that Osiris is blind because he is dead. The death reaches through the head; the lack of head means the lack of life, because it is impossible to see and breathe.
The hair over the face is a gesture with a deep symbolic meaning, it dives the mourner in the same blindness of the dead one, so to put that hair away from the face allows the mourner see and pass from the shadows of the decease to the brightness of the resurrection.
In Ancient Egypt the death was not the end of a human being. To die took part of the life. A dead person was not a disappeared person, but a transformed one. Dying was another step in life cycle, as it was in the other natural events: lunar and solar cycles, the annual flood, vegetation cycle…So, the burial was just a transition, the dead person was changing his condition. In funerals mourning women would cover their faces with their hair sema, reproducing the shadow in which was the deceased, but in the moment of the resurrection they would uncover them recreating the coming back to light.
Because the chaos is a « personification of the primitive vacuum, before the creation” and it becomes necessary to come back to it for finding the first manifestation of life, that in the funerary context will crystallize in the resurrection of the dead. The death is a return to the first moment of the creation, and in this new creation of revitalizing the deceased was crucial the life-giving gesture of shaking the hair.
 Chevalier et Gheerbrandt, 1969, p. 325.