A pump gag is a BDSM play staple. It is gear that's worn over the mouth to restrain the wearer (usually the bottom or submissive). It is different from a typical gag because it can be inflated to one's liking. Slow inflation can add to the excitement and feelings of helplessness. A pump gag is usually worn with an o-ring gag to keep it in place.
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Because it is inflatable, the dominant partner should remain aware of their partner's level of comfort. An inflatable pump gag increases the tension and tightness around the mouth. If it is pumped beyond the capacity of the wearer, it can make breathing difficult. Since safewords are not possible when wearing a gag, pay attention to your partner's gestures. Loosen the gag when necessary.