How is calculated the TWE score???

I took TOEFL twice, January 2004 and September 2004. My general score increased, but my TWE score decreased from 4,0 to 3,5.

My english skills were improved since the first TOEFL, but I have the feeling that the subject was an incident factor in the evaluations because in my first test was easy to talk about the "teaching techniques" (the subject), but in the second test was hard to explain in detail about "listening capability".

That why a want to know: How is calculated the TWE score?

Thank you for your answer.
(that text was wrote without any help)

Antón