The Story
When cooking burgers or steaks, if you flip them more than once you'll ruin them.
You should cook one side completely before flipping, otherwise the meat will lose most of its juices and flavour. You can though, rotate it, without flipping, to get cross-hatched grill marks.
Facts
Leaving meat continuously on one side for half the cooking time means that the meat surface becomes very hot and a large amount of the underlying layer becomes overcooked.
If meat is turned frequently, the surface still gets browned, but doesn't stay on the heat so long that the high heat penetrates deeply. The meat finishes cooking in a shorter time and less of the inside becomes overcooked.
(Note that for very thin meat, there is no point in flipping it at all. Simply leave it alone until the bottom is browned and the top is cooked. If you brown both sides, the top and bottom crusts will meet in the middle and the whole thing will be tough and dry.)
Why
Impatient people (i.e. almost everyone) naturally want to flip the meat as often as they can. Important people (i.e. self-styled experts) can demonstrate their superiority by showing that they have the self-control to resist flipping.