What is a "warp-spasm" in the novel, The Tain?

Cuchulainn undergoes what the translator translates as a "warp-spasm" either before certain battles, or in the midst of a battle to provide a boost to his powers. In the most noteworthy instance of the warp-spasm, his physical body literally becomes disfigured (his bones turn in his skin, his jaw slackens to reveal his lungs, his one eye buries itself in his head), making Cuchulainn a literal monster.