The EvaluateSentimentExtractor function uses test data to evaluate the precision and recall of the predictions output by the function ExtractSentiment. The precision and recall are affected by the model that ExtractSentiment uses; therefore, if you change the model, you must rerun EvaluateSentimentExtractor on the new predictions.
For basic information on precision and recall calculations see http://en.wikipedia.org/wiki/Precision_and_recall.