After reading your other post on swinging flat, I think I am leaning more toward what Shooting said, probably in your address, which is why I said, without video, it is very difficult to tell what you are doing. but I suspect the only reason you maintain your spine angle on the backswing is because you suck the club to far inside to maintain your balance from a faulty address position, that causes a flat swing.

At address there should be a vertical line from the balls (point between heels and toes) of the feet, up through the knee caps, then through the arm pits, all in a vertical line this is a balance position, from here you can swing without sucking the club inside to stay in balance, you will not feel the need to straighten up in either direction of the swing.

A check point for the backswing is to lay a club along the ball/target line off the right toe, when you begin the backswing halfway back when the club is parallel to the ground it should also be parallel to the club on the ground (directly above it) with the toe of the clubface pointing straight up. At this position you could stop turn your body 90 degree to the target line (back to target) without moving the club and simply drop it to the ground and be in the same relative position you were at address, square clubface and all. I suspect with your compensanting move to stay in balance rolling the club and pulling it in, if you tried this your clubface would be wide open and pointing to the right when you did the 90 degree turn to check. How ever far it off from what it should be at this point, you have have to make up for somewhere in the swing, difficult at best to do with any kind of consistancy.