One of the keys to back side surfing is "squaring your shoulders" to the wave.
Meaning, rotate your upper body so that the wave isn't behind you but beside you while you ride .
Of course your upper body will rotate back and forth as you ride, but a big flaw for many people is never rotating the upper body. An example of this is pig dog position, the waves side hand can drag in the face because the shoulders/upper body is rotated. This rotation often needs to often happen immediately upon pop-up or you just go DOWN to the bottom of the wave, which is fine if you want to do a bottom turn but not fine if you want to just drop into a mid-face line and start pumping from there..... the rotation also needs to happen if you ARE doing a bottom turn.
Yup Parko in that video surfing backside, watch his wave side hand and shoulder, it is frequently towards the wave.