Link could not use his spells because the Void Aura around his body interfered with the formation of his Dragon Power into spells.
In his body, however, his Dragon Power still circulated freely.
This gave Link an idea.
He held the Dragon King's Fury in his right hand and clenched a fist with his left, channeling the Dragon Power into it. Soon after, Link's left hand was clad in a crystal red light.
Originally, once the Dragon Power exited Link's body, it would naturally expand into a Dragon Power ball two feet wide. However, under the influence of the Void Aura, the Dragon Power could not even form a ball an inch wide before dispersing. This disruption completely prevented Link from casting any spells.
However, within that small space just around his fist, he could still use a minor support spell. Link used Clear Sign.