What replaces the thing.This thing may be an event or a subject...
What you said was wrong. Let's give an example:Your saying silly to him was wrong..Here what is used in the place of your saying silly .So, if you use your saying silly again it will be unnecessary.
It was wrong what you said. It ( the situation of your saying silly to him ) was wrong what(your saying silly to him) you said.
It would be ok if you told it is wrong to say that(your saying silly to him)
As for the other example,
It is pretty well understood what controls the flow of carbon dioxide in and out the atmosphere today . Here it and what are different things. It declares the stituation that something is understood by students.On the other hand what means the thing which controls the flow of...