Good point. My take on it was that he was similar to a Viking beserker, but exaggerated to supernatural levels, and once he was in his beserk trance he was capable of greater feats of strength. I do think it's linked to his ability to talk to spirits.
(I wonder why this mystery has never been discussed in this thread or the others dealing with earlier books?)
Same phenomenon is used in Pat Mills'
Slaine, who probably borrowed from the Tain, an Irish epic folkstory:
"he [Cú Chulainn] has his most spectacular ríastrad yet:
“ The first warp-spasm seized Cúchulainn, and made him into a monstrous thing, hideous and shapeless, unheard of. His shanks and his joints, every knuckle and angle and organ from head to foot, shook like a tree in the flood or a reed in the stream. His body made a furious twist inside his skin, so that his feet and shins switched to the rear and his heels and calves switched to the front... On his head the temple-sinews stretched to the nape of his neck, each mighty, immense, measureless knob as big as the head of a month-old child... he sucked one eye so deep into his head that a wild crane couldn't probe it onto his cheek out of the depths of his skull; the other eye fell out along his cheek. His mouth weirdly distorted: his cheek peeled back from his jaws until the gullet appeared, his lungs and his liver flapped in his mouth and throat, his lower jaw struck the upper a lion-killing blow, and fiery flakes large as a ram's fleece reached his mouth from his throat... The hair of his head twisted like the tange of a red thornbush stuck in a gap; if a royal apple tree with all its kingly fruit were shaken above him, scarce an apple would reach the ground but each would be spiked on a bristle of his hair as it stood up on his scalp with rage. ”
—Thomas Kinsella (translator), The Táin, Oxford University Press, 1969, pp. 150-153
He then attacks the army and kills hundreds, building walls of corpses. This 'ríastrad', translated as 'warp-spasm' (or for you Dutchies: 'kronkelkramp') seems very similar to what Logen experiences in his Bloody Nine persona.
Cheers,
Sfinx.