How far off am I on this simple reasoning? The shaft of light is emanating through the hole in the cloud from it's source, the sun. Very simply put, the shadow of the cloud is extending in exactly the same direction. The contrast between shadow and light is more pronounced and easily seen against darker background. As better explained above you are seeing light bounced off particles. In the case you observed there was enough difference in light illuminated particles (read sun coming through the hole) and un-illuminated particles (read portion of atmosphere in the shade of the cloud) to detect the difference with an un-aided eye even against the clear sky background. Willie, shade tree rocket scientist