There are several ways that shotokan Dojo practice this next move.
As you start to step, try not to over step with the body weight, once the body weight is just past the left leg, it stops and you rotate into kokutsu dachi (back stance).
We use a very small movement with the arms from Heian Yondans move 3 (juji uke). As you start the step, the arms move slightly to the left, the right arm rolls so the back of the fist is facing your face, then snap the arms, body and hip on completion of the moroto uchi uke.
There are several applications for moroto uchi uke, we like the block and attack applications, so the fist near the elbow, turns into a strike as the right arm blocks, but of course, there are many other applications forthis move.