Switch
From wipipedia.org
In BDSM, a switch is someone who participates in BDSM activities as both a top and a bottom. A switch will be the top on some occasions and the bottom on other occasions. Switches are very common; partners may switch roles based on mood, desire, or being with a different partner.
A 'switch' may be in a relationship with someone of the same primary BDSM orientation (two dominants, say), so switching provides each partner with an opportunity to realize his or her unsatisfied BDSM needs with others.
A person who engages in self-bondage can be viewed as taking both roles simultaneously.