Ignasi,
The "seen from behind" refers to the positioning of the right shoulder of the Emperor. The
Head remains in profile.
I will illustrate with a couple of
coins of Probus:-
This
bust seen from the front.
This one seen from the rear.
On the first coin the
cuirass can be see on the left hand
side of the coin and you can see the chest of the emperor. On the second coin the
cuirass can be seen towards the right and what we can see is the back of the emperor.
(Mauseus beat me to the post by a few seconds but I thought I would post it regardless.)
Regards,
Martin