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The unit turn is the action of turning your “unit”, your torso to the side in preparation for your shot.
It is performed simultaneously with the first step; the first step is a shift in body weight to the outside leg that provides a stable base of support, allowing you to coil back, and load your obliques by creating an offset between your hips and shoulder.
The non-dominant arm assists in turning the shoulder around by supporting and pushing the racket back, increasing the speed of your swing and giving you a longer and more powerful swing path.
The preparation is very similar on the double-handed backhand, loading into the outside leg for stability, turning the shoulders around, and uncoiling through your shot.
On the single-handed backhand, it is the inside foot that steps across into a closed stance and maintains grounding while the non-dominant hand pulls the racket back to optimize rotation.