Definition - What does Pump Gag mean?
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.
Kinkly explains Pump Gag
Because it is inflatable, the dominant partner should remain aware of his or her 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, to say the least. Since safe words are not possible when wearing a gag, pay attention to the partner's gestures. Loosen the gag when necessary.