Left handed watches are essentially mirror flipped versions of a "traditional watch" designed to be worn on the right wrist. This mirror image would place all of the controls at the same place of the opposite wrist, so a left-handed wearer could enjoy the same fit, feel, and function as a right handed wearer using a traditional watch. The first production left-handed watch was produced by Panerai in 1998.
Fun Fact: the name Panerai chose for this style of watch is Destro, which means right.