Topping from the Bottom is a term used to describe the potential reversal of roles in a BDSM scene. It is not seen as a desirable situation.
In a Topping from the Bottom situation, the submissive attempts to manipulate or control the actions of the Dominant through verbal or physical cues. In typical BDSM relationships, it is understood that the Dominant partner is in charge of all aspects of a play session or scene, and the submissive has agreed to follow their desires and directions. Topping from the Bottom breaks away from that protocol and can make the scene difficult for the Dominant.
It is different from bratting, where the sub is willfully obstinate and must be brought to hand by the Dom as part of the agreed upon scene. Topping from the bottom refers to the sub not conforming to the agreed upon roles.
BDSM scenes, and the assigning of roles as Top and bottom can be very particular to each person. Some people only identify as a Top or Dominant, and some people only identify as a bottom or submissive. Some people also identify as Switch, meaning they can identify as either a Top or a bottom, depending on their desire at the time.
BDSM relationships are enjoyed by people of all sexualities and genders, so those factors do not factor into issues relating to Topping from the Bottom.
Typically, before commencing a BDSM scene, players will agree and consent to their identities and roles in the scene. For many, a significant part of enjoying the scene is relishing these roles and the power dynamics agreed upon.
Some of the ways someone can attempt to Top from the Bottom are: offering verbal cues that change the dynamics and control the play; suggesting different types of play mid-scene; moving their body in ways to guide physical touch; and, resisting cues from the Dominant.
The undesirability of Topping from the Bottom should not be confused with issues involving consent. Anyone can choose to remove themselves from a BDSM or sex scene from anytime and that should be respected without question from the Top, this is a separate issue from a sub disrupting the power dynamics in an negotiated scene .