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The ergonomic shape keeps it in place, even during vigorous use. Once inside, the middle tunnel is ideally sized to be fucked, lined with ribbing and designed to caress the underside of your cock head with every thrust.
Use it during extended ass play to avoid chafing and discomfort, to tighten up loose holes, or as a control device. Also great for enema play or internal examinations. You are only limited by your imagination.