In our Dojo also, the only "back" step is the "tenshin" step. Two types of stepping are Tsugi-ashi and Ayumi-ashi. When you enter in Irimi, your first step is always in Tsugi-ashi; and when you enter in tenkan, our first step is done in Ayumi-ashi.
Irimi and Tenkan are the polarization ofthe defense. Irimi is Yang and Tenkan is Yin.
Ura and Omote are the polarization of the technique. Omote is Yang and Ura is Yin.
You can thus combine the two polarities to create four movements. Yang- Yang, Yin-Yin, Yang-Yin and Yin-Yang. In techniques, it means Irimi-Omote, Tenkan-Ura, Irimi-Ura and Tenkan-Omote. When you can add the Uchi and Soto polarity, you have 3 polarity. Thus you can create 8 movements now. (Uchi is Yang and Soto is Yin).
NB : the referencial point of Yin-Yang, in our Dojo, is the physic. Follow the teachng of G.Ohsawa (his real name Nyoichi Sakurasawa). Unlike the metaphysical point of vue as in Acupuncture or Taoism.