Did you see the word Hagzissa from this article:
I've not really seen it used outside of that context, and never in my own practice. It's a German word which is almost akin to "witch", and not a real practice as far as I know- though I could be wrong.
Anywho: Seidhr is a type of magick that most Volva practiced, though they also did some other things like basic energy work and the like. Seidhr is very similar to shamanism in several ways, and they tend to overlap. It's hard to give a short definition of the practice. It often involves things such as altered states, trance-work, divination and many more practices combined into one. It is a type of magick that rarely dealt with the runes at all. Instead, it was more centered around the spiritual aspect of things.
The magick of Seidr as described by Edred Thorsson and Freya Aswynn, includes divination, soul travel, shape shifting, necromancy and cursing.( Ed Richardson)
I'm always willing to chat and share what I know about the practice, so mail me if you want.
Here's some reading I would suggest for you:
The Book of Seidr: The Native English and Northern European Shamanic Tradition by Runic John
Trance-Portation: Learning to Navigate the Inner World by Diana L. Paxson
Volva Stav Manual by Kari Tauring
Seidr: The Gate is Open by Katie Gerrard