This is a sexual term referring to a particular sex act where a man receives a blow job from a woman, cums in her mouth, and then the woman (or could be a man) pretends to swallow, then turns to embrace her partner in a passionate kiss, thereby open-mouth kissing him with his own cum in her mouth. Hence, San Francisco Switcheroo (almost gay, not quite)
Man: You gonna swallow right?
Woman: Of course
Man: You better not pull a San Francisco Switcheroo on me cause I'm not okay with swallowing my own cum.
When your partner has fallen asleep after sex, quietly slip out and exchange places with a friend who's the same sex as your partner (or the opposite sex if you're homosexual). After they've gotten comfortable together, call your partner or knock on the window. Surprise!
I donkey punched her the other day and she got me back by pulling a San Francisco Switcheroo on me.