Source : Free On-Line Dictionary of Computing

under the hood
     
        [hot-rodder talk] 1. The underlying implementation of a
        product (hardware, software, or idea).  Implies that the
        implementation is not intuitively obvious from the appearance,
        but the speaker is about to enable the listener to {grok} it.
        "Let's now look under the hood to see how ...."
     
        2. Can also imply that the implementation is much simpler than
        the appearance would indicate: "Under the hood, we are just
        fork/execing the shell."
     
        3. Inside a chassis, as in "Under the hood, this baby has a
        40MHz 68030!"
     
        [{Jargon File}]