A breast pump is a pump that vacuums the breasts . This one vacuum pump is especially popular among BDSM people. The breast pump ensures that the breasts swell and become larger. The Master or Mistress has control of the pump and can thus torment the sub. A pump like this is not only used by SM enthusiasts but also by women who would like to see their cup size increase. The breast pumps are placed over both breasts . With a hand pump you suck air into the cup (s), causing your breasts to swell. In addition to pumps for the entire breast, there are also models that are only intended for nipples, these are so-called nipple suckers.