Zinn has a great book with a chapter on this. Here's a quick summary: take a road position, then rotate it forward around the axis of the pedals until you get in an ideal aero position, then you have the tri aero position, or TT position with aero bars. Some good side effects for saving muscles, but done right it spins the rider fwd to the only major change is gravity, not position relative to the pedals.
There's also the SLAM position, which has some differences.
Keep in mind that these things are hotly debated, so take it all with a grain of salt.