An inflatable gag is a rubber balloon (often attached to a leather plate to hold it in place) with a valve and a pump. Once inserted into someone's mouth, it is inflated to fill the mouth in much the same way as a ball gag. It is very difficult to eject an inflated gag, but there may be a strap to fasten it round the head and secure it further.
Inflatable gags are very effective in silencing the wearer but they also block the airway and for this reason the gagged person should never be left alone while wearing it. It is possible to have the inflatable part with a hole in it, making it a sort of tube gag; such gags are safer but less effective.