The hama typically took on the appearance of an animal to those with second sight. The hama existed in our realm, tethered to the human body, while simultaneously existing in the Spirit Realm. Saying ones name during trance could result in the hama abruptly returning to its corporeal body.

“[Lodmund] commanded that no one should have the audacity to utter his name- because his inner shape would be leaving him to fulfill his desire.”
–Ibid

There are certain risks associated with hamafōr. The hama can become trapped in the Otherworld, unable to return to its body. In Hávamál , Wōden speaks of a charm able to prevent witches from returning to their skins.

“If I see witches
Riding through the air,
I cast a spell
So they get lost
Without finding their skin again
Without finding their spirit again”

Ones hama could grow or wane depending on specific events. In Hauksbók, Odd kills and eats a bear which ate his father and brother, thus absorbing the collected vigour from all three.

“Subsequently, Odd became wicked and a hard person to deal with. His power to shapeshift was so strong that one evening he left his home in Hraunhofn and reached Thjorsardal the following morning- a distance of two hundred fifty miles!- to aid his sister, whom the folk of Thjorsardal wanted to stone to death for sorcery and magic.”

In The Return of the Dead, Claude Lecouteux theorizes that the hama is man’s vital essence and is the part of the being which lingers in the mound post-death. Even the smallest portion of human matter can contain this hama and there are stories of hauntings continuing when ashes are eaten by a foraging animal, echoing the above mentioned tale of Odd and the bear.

Further Reading:

The Return of the Dead: Ghosts, Ancestors, and the Transparent Veil of the Pagan Mind by Claude Lecouteux