The Switch (BDSM) reference article from the English Wikipedia on 24-Apr-2004
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Switch (BDSM)

Sponsorship the way you would do it
A switch is someone who participates in BDSM as both a top and a bottom. Switches are very common; partners may switch roles based on mood, desire, or being with a different partner. They might also switch because both partners in a relationship are (for instance) bottoms, and they will take turns at 'switching' to top so that they can continue their activities.