The "handshake" (thumb up, pinky down) position represents the strongest and most stress-free orientation of the hands on the handlebars.
This corresponds to the hoods of drop bars or the horns of bullhorns.

Sit in a hard chair with your feet aligned as if on pedals and about as far apart as on pedals.
Hold a pencil in each hand.
Lean forward and imagine holding your handlebars about as far from your hips as suits you; and begin to stand up.
Just as your bottom begins to break with the chair, look at the pencils in your hands.
The angle of the pencils indicates the angle of the hoods or horns, and the angle of your torso (as determined by the height of the bars and the length of the stem), best suited to YOUR body.