With only a small use of magic, such as the one possessed by sparks, a mortal can activate the hamn and take on features of the animal it's from. A full bear pelt hamn would allow the mortal to transform into a bear while a wolf's paw would give them claws. The magical power of the hamns drain over time, however, and need to be recharged with blood in a spiritual ritual. Hamns used by successful fighters tend to be self-sustaining though, as they often get covered in the blood of their enemies, some of which who may have been vessels.
Havsgardian Vessels on their pilgrimage are often asked to recharge hamns in exchange for food and lodging as they pass through villages.