When you call TTree::Draw, a TTreeFormula is being generated. You can trace back the C++ expressions behind the scenes in the TTreePlayer.cxx and TTreeFormula.cxx files:
https://root.cern.ch/doc/master/TTreePlayer_8cxx_source.html#l00333
https://root.cern.ch/doc/master/TTreeFormula_8cxx_source.html#l02674
https://root.cern.ch/doc/master/TTreeFormula_8cxx_source.html#l03925
See some bug of TTree::Draw: https://sft.its.cern.ch/jira/browse/ROOT-8009
An alternative: https://root.cern.ch/doc/v608/classTTreeReader.html
A newer more robust method: Selecting subset of entries in TTree and plot them in Histogram