Bit gag
From Voyeurweb's Wiki about Sex
The bit gag is a type of gag and sometimes used in BDSM play. It is styled like the bit for a horse. It is mostly used in so-called pony games. A bar is strapped into the mouth behind the teeth, with the straps going around the head to secure it in place. The bar is usually made out of rubber or metal. The main reason for using a bit gag is probably the visual effect as the bit gag does not prevent her from speaking.
Photos of Bit Gags
