under the hood

under the hood [hot-rodder talk]  1. Used to introduce 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!"