Ernie knew about the Cruciatus Curse. He knew how excruciating it was supposed to be, and he knew that the Carrows had been using it on students. But he realized that there was no understanding it, really, until you were experiencing it.
It was the most excruciating thing he'd ever experienced, and his thoughts were nowhere at all except on the pain he was feeling. He had no sense of anything, and the fact that he was no longer standing but instead on the floor went entirely unnoticed. He was too distracted by the Blinding. White. Hot. Pain.