When having sexual intercouse and one switches from the vagina to the anus quickly and then looking at your penis to judge if analingus is safe.
"I can't beleive he tried a shoff swap on me, but when it came out clean he made me forget my concern."
The switching and/or swapping of microwaves, even when they aren't the same kind :o)
(After taking back a microwave and trading it for a different brand and much better one...)
"We just pulled a Shoff swap!"