In Saxo's re-telling of a portion of Hrólfs saga kraka and the event in which the vǫlva Heiðr uses seiðr in order to find the two boys Helgi and Hróarr, he elaborates upon the supposed skills of the practitioner.
Saxo tells that due to the power of her incantations, the vǫlva was able to "summon to her hands something in the distance, visible only to her alone, even when it was tied up tightly with knots".
The motive here does not appear to be solely literal due to the multiple functions that knots held within Viking society. An person's mind could be tied in knots ‑ befuddled or misdirected ‑ and the condition was associated with the practice of sorcery. Thus it could be inferred that even the information held within an individuals mind could be divined however well they believed it to be hidden.
Davidson further connects this theme of knots and sorcery with the god Óðinn as it is generally accepted that knots would have been associated with the god, as can be exemplified through the depictions of the valknut, his own sorcerous powers of confusion and knowledge divination as well as through ritual hanging victims.
See Saxo Grammaticus. Davidson, H. E (ed.) and Fisher, Peter (trans.) Gesta Danorum Books I-IX (D.S. Brewer, Cambridge 1996), p202.
(Contributed by Douglas Robert Dutton.)
Main text: Gesta Danorum
Text sections: [not skaldic]