Relative Clauses

She looked as if she needed a rest.

I have an English grammar book that cites as if she needed a rest as an example of a relative adverb clause. Here is the explanation:

As if she needed a rest tells how she looked.

What's wrong with that?

 Looked is a linking verb. To use a linking verb is to rename or describe the subject, adjectives describe nouns, therefore as if she needed a rest cannot be adverbial. If you are going to allow it to be adverbial, then in a sentence like I felt sick, sick is adverbial, because it tells how I felt.

Wait a minute, I was taught that to test for a linking verb, you can replace the verb with "to be" or "=" and if the meaning does not change, then it is a linking verb.

She = as if she needed a rest
That really doesn't work here.

The reason it does not work is because even though the clause is adjectival, it is not functioning as a predicate adjective (it is not a subject complement). If you want to know why, go here: http://livefullyawake.blogspot.com/2011/11/when-just-because-is-not-satisfactory.html

I will not name the text because overall, it is a reliable source and I do not wish to discredit it. But this demonstrates how engrained it is for us to classify words by what they tell us rather than by what they modify. 

But, couldn't as if she needed a rest be adverbial in this sentence?

Only if she looked with her eyes.