[All]: This helmingr is a subordinate clause so the full sense of the complete stanza can only be guessed at. There are two ways of construing the clause: as is done here, in Skj B and SnE 1998, and as suggested by Kock (NN §2989H). Kock takes élreifar dœtr Ægis ‘the storm-happy daughters of Ægir’ as the subject of the clause, with harðar rokur ‘hard whirlwinds’ as direct object. This gives the sense ‘when the storm-happy daughters of Ægir [WAVES] wove and tore apart hard whirlwinds, nourished by frost, from the white mountain range’. Grammatically, it is unexceptional, but it seems to be less good from the point of view of sense.