As far as I understand it, to maintain facing, you can only "step" a specific direction to balance an upper body rotation.
And as far as I understand, that is true in anything the human body does. All WC, including Ving Tsun, includes rotation.
This is true. However, as I noted before, I think there are two "schools of thought" here. This applies to the rotation/pivoting in Chum Kiu as well. To make it easier to write I will refer to the 1st school of thought I mentioned before.....rotating to deflect and generate power...as "rotators." And I will refer to the 2nd school of thought...driving in a wedge shape to deflect and generate power...as "wedgers." No derogatory meanings are intended!
"Wedgers" most certainly use rotation. But they use it a bit differently than "rotators" do. Picture standing in a ready stance with the hands out in front. The front hand or "Man Sao" hand is like the front site on a gun. It is used to aim directly at the opponent's center of mass. The "front site" also stays on the center directly in front of the person doing it aligned at a 90 angle with the shoulders. Any rotation used by a "wedger" is primary to align the "sites of the gun"....like pivoting the turret of a tank to bring the cannon barrel in line with the target. So once the "wedger" has placed his sites "on target" by rotating his stance or stepping, he drives the wedge in with footwork generating translational force in his punch. If he meets an attack, he will "wedge in" to deflect as he moves into the opponent. He uses defensive hand forms like Tan Sao as part of the "wedge" to deflect as he goes in.
A "rotator" approaches it a bit differently. The "sites of the gun" do not stay directly out in front aligned 90 degrees with the shoulders. The alignment of the arms are allowed to swing a little off the 90 degree line in either direction. When he pivots, rather than the "sites of the gun" staying on the 90 line and moving with the pivot, the "sites" may actually stay in the same place as the rest of the body moves. So now, the "front site" or Man Sao hand is going to go forward as the person pivots rather than swinging around like the turret of a tank. So rather than "wedging in" with a translational force, he will pivot to drive a punch in with rotational force. If he meets an attack, he will pivot so that the shoulder moving back deflects with a defensive hand form like Tan Sao while the other shoulder moves forward with the strike.
So you see, both approaches use plenty of pivoting or rotating. But they apply it a little differently. And as I said before, what I described would not be exclusive to either school of thought. "Wedgers" will still use the pivot to deflect defensively at times, and "rotators" will still drive in like a wedge at times. Its a matter of preference or emphasis.
I hope all that makes at least a little bit of sense!