Review

class CustomScoreQuery.CustomScorer {
    /*(non-Javadoc) @see org.apache.lucene.search.Scorer#explain(int) */
    public Explanation explain(int doc) throws IOException {
      Explanation subQueryExpl = weight.subQueryWeight.explain(reader,doc);
      if (!subQueryExpl.isMatch()) {
        return subQueryExpl;
      }
      // match
      Explanation[] valSrcExpls = new Explanation[valSrcScorers.length];
      for(int i = 0; i < valSrcScorers.length; i++) {
        valSrcExpls[i] = valSrcScorers[i].explain(doc);
      }
      Explanation customExp = customExplain(doc,subQueryExpl,valSrcExpls);
      float sc = qWeight * customExp.getValue();
      Explanation res = new ComplexExplanation(
        true, sc, CustomScoreQuery.this.toString() + ", product of:");
      res.addDetail(customExp);
      res.addDetail(new Explanation(qWeight, "queryBoost")); // actually using the q boost as q weight (== weight value)
      return res;
    }

}