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The emoticon that shows the lips being tugged forward on the backwards pull whilst sucking on something, as seen from an overhead or semi-overhead viewpoint. Typically this means fellatio, but it can also be seen when a character is sucking on a more innocuous object, such as a popsicle or pen.
The equivalent pixiv tags are ひょっとこフェラ (Hyottoko fellatio) or バキュームフェラ (Vacuum fellatio).