title: How does SelectFromModel() work from from_model.py?
body: <pre><code>fsel = ske.ExtraTreesClassifier().fit(X, y)
  
  model = SelectFromModel(fsel, prefit=True)
  </code></pre>
  
  <p>I am trying to train a data set over the <strong>ExtraTreesClassifier</strong> How does the function  <strong>SelectFromModel()</strong> decide the importance value and what does it return?</p>