That's a slight misrepresentation of ABX testing.
ABX provides the subject with an example of A, an example of B, and then an unknown (X) where the subject then tries to decide of X was A or B. So there really isn't a "false switch" as part of ABX testing as you're not going back and forward between A and B but trying to determine what X is.
ABX testing is used when you are trying to ascertain whether or not A and B are discernibly different while A/B is most useful for preference.