Well, at first I wondered if it might have been a translation error, and they meant grated taro; raw taro root contains calcium oxalate crystals which are dangerous if consumed (being basically the same thing as eating ground glass). However, I looked up the Japanese yam, and it appears that it also contains oxalate crystals. These are typically eliminated by treating the yam with vinegar. But yes, the ground yam and its juice, unprocessed, can cause severe itching upon contact with the skin.