When does the rule "only flip meat once" not apply?
I believe the goal is to get the best sear possible on each side of the meat. Moving the meat around too much could cause it to cook completely without getting a good sear on the outside.

Clearly this doesn't apply when doing anything "rotisserie" style, as you're trying to do the exact opposite: slowly cook the interior of the meat without burning the outside to a crisp or unevenly.

Also, see this similar question: Grilling burgers: flip once, or keep flipping?