Someone would use a clit pump primarily for sensation. Suction stimulates the clitoris, bringing blood into it, which causes it to engorge. The sensation of suction on the clitoris can feel very erotic, and if you squeeze the pump rhythmically it can feel something like oral sex. It can be used solo or shared with a partner. For some women, its use could result in orgasm. The cylinder or dome goes over the clitoris. The edges should contact the skin all the way around (sometimes you have to press it into the mons/labia tissue a bit to get it to "seat"--also, some women trim or shave their public hair to help with this, but really unless the hair is very copious it can just be brushed out of the way with one's fingers). Keeping that contact, begin to pump. Most pumps have a valve that allows the user to get just suction (where it sucks tighter and tighter against the body, and a vacuum is created inside), or light suction so that with repeated squeezing of the pump, you get more of a "suck... suck... suck" sensation.