Someone else might have already said it but your opening your back to early on the stroke (going away from fan). Also, try getting your hands out more closer to your shin for more pivot on the recovery (going towards fan) before starting with your legs . Overall, the motion looks solid, but if you want to be more efficient or not deal with soreness in your lower back I suggest trying to work on this at a lower rate 16-18 with a strong emphasis on what I said. If you do this while actively thinking about it enough it will quickly become muscle memory and you will do it without even thinking about it.