To any involved in a power exchange trust is often discussed a great deal. It is the grease on the wheel that allows the energy to free flow between the Dominant and submissive parts. Without reliance that mechanism freezes up and requires repair.
The bottom must be able to rely on the Top. The Top must also have confidence that the bottom trusts them. Without those elements there is no real exchange of power and what remains verges on make-believe.
It sounds harsh, the make-believe bit, because I am certain there are some who would argue their dynamic is not fantasy.