Ivan, many teachers, and schools approach the twist differently. I try and minimize the use of the legs in general, and put the focus on the trunk and of course, control of the shoulder. I am trying to "de-emphasize" the back foot and leg, so the balance hinges almost entirely on one arm and one leg (in terms of the limbs). It is less an active plantarflexion and more a comfortable position for the foot to sit and be as little involved as possible. Dorsiflexion would involve the foot, and probably the leg, more. As for the work in the trunk, it should be the focus of the exercise. Not the plantarflexion or dorsiflexion of the foot, which is an important detail but certainly not the essence of the exercise. Enjoy!

Rael Isacowitz  Thank you very much for your thorough and detailed answer! Will be trying it both ways in my upcoming practices!