As far as I can tell, some Asus 3xxx cards have red leds right next to the pcie power plugs. They illuminate whenever the card detects a power issue with one or more of the cables. Some people have been getting the red leds even though they are experiencing no other problems or issues. I guess Asus added a switch to turn off those leds, for those individual applications who have no other issues. I had the same question, because it's not well documented and there is no tool tip.