so everybody is writing about keeping your head up and than i watch both links and this guy's head is not up at all... the mystery of the back kick - to be continued.
These are not very good examples of how to do the back kick.
While learning to Back kick you need to keep your head up, this helps you stay trimmed. You need to be horizontally trimmed. Don't drop your knees. The power comes from the lower leg, rather than the fin itself, the Fins really are just getting in the way.
It's a 3 stage process.
step 1, Knees bent at 90deg, feet parellel to floor, Soles of the feet together (or at least touching), this stiffens up the fins, as they are pushing against each other.
Step2, Slowly extend the lower leg backwards, while making sure the feet stay parellel to the floor.
Step3, Pull your lower legs back towards the body, stopping at about 90deg, while twisting the angles so the feet go outwards
Then slowly return to step1
This is very hard to explain in text, but fairly easy to show in the water, it's one of those skills that really needs instructor time to teach properly.
HTH
John