I don't quite understand how these work.. Let me get this straight, this unit fits over the lens and is translucent enough that light gets through, plus it is white in color, therefore you have basically have a white card right in front of the lens, is this correct?

Who is to say that thje color temp of the light hitting the back of this unit is the same as at the subject? Wouldn't lens hoods interfere with this? Why not just drop a white card somewhere in the area of the scene you will be shooting (or find something white like snow)? This seems like a much more accurate and only slightly harder way of doing the same thing.

If the sun is at your back, with another light source at the front and you use one of these while the lens is pointed at the scene, it seems like it would be way off.