I understand you. For me it's slightly different. Sometimes I may want the gal to take the leading role in sex and be the cowgirl or whatever. Maybe even tie me (else I know my nature won't allow me to just lay back and enjoy) or even some dommey talk (eg come here little boy). But I don't see this as changing roles because ultimate I have the control and power. It's more like her servicing me for a change. Not quite what you mean I know but it is a sort of halfway.