There are two distinctly different ways to do the back kick and helicopter turn. I worked on the technique I was originally taught for years before being introduced to the other. The technique I used to use requires a twist of the foot to put the flat surface of the fin into position to be the propelling surface. The other keeps the fins flat and uses the sidewalls of the fins as the propelling surface. The second method is much easier to learn.From that you develop the helicopter turn which is kind of one leg doing a frog kick, the other doing a back kick.
The back kick is the hardest to master and by far the weakest kick. Arguably it's one of the most useful to stop your forward motion and manoever backwards.
I don't want to turn this into a seminar on kicking techniques; I just recommend that if you seek out instructional videos or personal instruction, you keep this difference in mind.