Basics
Unlike the vagina, which is closed off by the cervix, the rectum leads to the sigmoid colon. Objects that are inserted into the rectum can therefore potentially travel up into the bowel: the flared end on a butt plug exists to prevent this. Many dildos lack a flared end, and thus it is ill-advised to use such dildos anally, since they may get stuck, requiring medical extraction.
In addition, the lower bowel above the rectum is easily perforated. For this reason, butt plugs tend to be shorter than dildos, and their marked size generally indicates the circumference of the device rather than the length. They must also be very smooth to avoid damaging the rectum or bowel. In order to get them into the rectum, they most commonly have a general profile of a round-ended cone which then narrows to a "waist" which locates itself at the anal sphincter, with the flared part outside the body, preventing the butt plug from slipping further into the body. The sphincter muscle will hold the plug in place by the waist, preventing the plug from slipping out unintentionally.
As with other activities involving anal penetration, such as anal sex, large amounts of sexual lubricant and a slow gentle approach are needed to insert or remove a butt plug.
Butt plugs are sometimes used covered by condoms for hygiene, and to allow for the easy disposal of any feces with which they may come into contact. Nevertheless, they should not be shared with other people, due to the risk of blood-borne diseases, including HIV that can arise from the transfer of body fluids from one person to another. A butt plug isn't necessarily confined to only anal stimulation; butt plugs may be used in any orifice in which insertion is physically possible.
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