[shopsieupet]

The thrower establishes his pivot point in his left foot (in bounds). Then, with his other foot steps in an out-of-bounds area and make the throw. Is it legitimate, or does he has to have all of his body inside the playing field in order to attempt a legitimate throw?

[Rueben Berg]

This is allowed, as per 11.3.2. :

A thrower in possession of the disc, who contacts the playing field and then touches an out-of-bounds area, is still considered in-bounds.