The instructions in order are to draw the shoulder blades down the back (towards the waist), turn the upper arms from front to back in order to clinch the outer shoulder blades in towards the spine. Then tighten the arms by brining the triceps to the bone. All of these actions apply in Trikonasana to extend the chest.
To turn the chest, move the energy of the bottom shoulder blade and arm towards the floor as you fill (like a balloon) the opposite side upper chest.
To turn the head, bring the top outer shoulder blade down towards the floor and away from your ear.