Is is true or an urban legend that if you whisk the opposite way while making mayonnaise (after you've whisked in one direction for a while), the mixture will be "undone" into its components?
Welcome to the world of urban legends and old wives' tales.
Handmade mayonnaise can be a fickle thing to create if you don't work within the laws of physics and chemistry and don't achieve the desired emulsion. So like with other tricky processes, many "rules" have developed, that are more myth than method. (I was even told once to "always stir counter-clockwise"...)
If you think closely, it becomes obvious that an emulsion - for mayonnaise, tiny particles of oil suspended in water - is something completely different from tiny strands of something. One can not "unwind" or separate by stirring in the opposite direction.
My personal theory for the origin of the "don't change direction" rule is that if you constantly stir in one direction, you reach some kind of "flow state" and work in a steady rythm - the latter being far more important for a good whipping or stirring result than any direction. And that the rule predates the use of electric kitchen equipment, going back to the times where all whipping was done by hand.
This is an old chef's tale. The important things when making the emulsion that is mayonnaise are that the oil is evenly dispersed and that it is broken into the smallest possible droplets. The direction of whisking has no effect on those factors.
In fact, I find that a back-and-forth (zig-zag) whisking motion works best, especially when getting the mayonnaise started. This lets the utensil continually hit and break up the oil, mixing it much more thoroughly than circling around the bowl.