Very Tired or sleepy. The action footballers make when they score headers. The nod down to head the ball into the net. Similar to nodding asleep on a train.
Non maintenance of the cooling system that results in a blown head gasket, antifreeze leaks on to hot exhaust manifold and creates steam. When the antifreeze mixes with the oil it has the color and consistency of flan, thus, the term; “flanning the headers” is applicable.