Someone who enjoys anal sex, but unlike most, is the one in charge, directing the action and setting the tone.
In most, the bottom is considered the passive party to the sexual activity, however there are those who are actually the receiver, who are in control of the moment.
In BDSM, a power bottom, can be a dominant partner, directing their 'submissive' to penetrate them, giving full and explicit directions on how to penetrate, how to pump, or other directions, so that the 'dominant' is pleased.
The use of 'power' usually refers to who is in control, in sexual parlances.
Noteworthy: In some cases, upon penetration, there can be a 'switch' of power, of control, from the top to the bottom. (see Switch) This change, can occur during the session, and can happen several times, though usually it is agreed upon prior to the sex session.