How about this take on it. I don't think a person wants to "switch" so much as just experience more. A Master/Mistress is always in control, but they might want to experience the rush of being dominated once in a while.

Try this...

If you let your slave/sub have a night of fun, first lock them in a chastity belt. She might take a strap-on and take her Master, but she won't be able to receive any pleasure until he gets the key.