Oscar says the same thing on his website:
After you have this movement well grooved in, get someone to toss gentle balls to your backhand. Lift the ball well over the net and finish all the way, perhaps exaggerating the lift, and getting your balance by pulling away from the ball and up. Especially if you are too close to the ball, pulling back will give you plenty of room to swing, with your arm extending towards the target and then across towards the right.
This type movement combination has been shunned by conventional tennis teaching, that tells you to step forward into the hit and stay down. This, unfortunately, destroys the natural acceleration of the arm. Try it both ways, and you will notice the difference.
I have a feeling this is again one of those legacy items from the transition period. In the wood era, it was necessary to stay down and step into the ball to get any meaningful power. Today, rackets are more powerful, and also, as Federer discovered against Nadal, high-bouncing slow balls are common to the backhand even on grass and hard courts. So, while there is a classic video of Federer with his back foot almost kneeling on the ground, he also hits backhands in a more upright position. So it is no longer necessary to adhere to the stay down advice very strictly. He could have just said that, instead of saying the other things like going backward. He confuses the finish which will result in the body rising with a backward movement of the body during the hit, similar to the confusion on the forehand about pulling away before impact instead of the reality of finishing the swing after hitting solidly through the ball.