A special type of hook that is hooked under the nostrils then used to pull the nose upwards.
From the Yuribou Hentai Dictionary:
顔面攻撃 (ganmen kōgeki), lit. "face attack". A variation on S/M play in which the captive girl's face is contorted to make it look ugly so she is embarrassed and feels dominated, and the men feel domineering and virile. Commonly used are fingers in nostrils or mouth, or retractor devices, attached to a headpiece or collar, which hold open the mouth or nostrils. The nose-specific version is called "hana-zeme" (鼻責め "nose play"), and the retractor devices for it are "hana hukku" (鼻フッウ "nose hook").
