Take another look at the original sentence:
You wanted to fix it this way:
To add a comma and the conjunctive adverb therefore between volunteer and whenever would create an equally bad problem, a comma splice. A comma splice occurs when you have two complete sentences joined with a comma alone. Tracy always knows the right answer in English class but is too shy to volunteer is the first complete sentence. Whenever Miss Simmons asks a question, Tracy's hand feels ... begins the second complete sentence. The spot between volunteer and whenever needs a stronger break than a wimpy comma and a mere transitional word.
You might want to consult the rules for fixing comma splices and fused sentences.