Board Thread:Speculation House/@comment-26822171-20151216132821/@comment-27121579-20160104005051

Well now we do know that eyes have got something to do with it. Amber's eyes glowed a brilliant fiery gold when using her powers, and Cinder, once using the Grimm bug, covered Amber's left eye, leaving only Amber's right eye to glow. The power was then transferred and you can see Cinder's right eye (i think it is) glowing, showing that the process is half complete when Qrow cuts it off.

So it's definitely something to do with the eyes - but not in the way you were thinking.