For example, during Flogging, the person swinging the flogger implement would be the top, while the person being struck with the flogger would be the bottom.
The term bottom originates from a more general use of the word, especially in the gay male community, to mean receptive partner during anal sex.
It is very important to note that this role does not necessarily involve any form of Power exchange but, rather an exchange of some form of sensation or service, however in many cases regarding BDSM the role of bottom will be partaken by some form of s-type that does negotiate for some form of power exchange. It is further important to note that a D-type may also bottom for a Scene, an example might be a Master bottoming for a massage from their slave.
A demo bottom, sometimes called a demo dolly, is someone that is bottoming for the purposes of a Demonstration. This person is generally selected for their extensive experience and comfort with the activity being demonstrated as well as their comfort with the person performing the duty to ensure that nothing unexpected occurs to ruin the demonstration for those who are there to learn. In many cases they will be an optimal specimen to show the activity being demonstrated in the case various parameters might exist to that end.
A performance bottom, is much like a demo bottom, but far more specialized and indulgent in the areas of showmanship. The performance bottom serves as an penoptimal presentation piece and instead of merely demonstrating, the performance bottom is there to assist in putting on a show as an act of exhibitionism. This requires a special skill set to generally indulge in making the top for the scene look amazingly skilled, refined, and talented, the parameters of which will change drastically depending upon what the type of demonstration is being done. Many of these performance bottom techniques will require intense levels of experience and comfort with the top and the activities to take place, and will often require a lot of rehearsing and taking direction as well.