The Woodchop is one of the fundamental transverse plane/rotation exercises.
Here’s how you do it:
Grab the handle with both hands and stand sideways relative to the cable. Assume an athletic stance with your knees slightly bent. Keeping your torso upright, bring your hands down and across, toward your far hip.
(Although our model slips a little in one of her reps …) Be sure to keep your torso upright throughout the exercise. You may feel a tendency to lean inward toward the machine at the top of the motion (the mistake you may see in the video) and a tendency to crunch at the top of the motion.
Remember that the goal of this core exercise is to keep your core relatively stable against the diagonal resistance provided by the cable.
Also, be sure to pivot on your inside foot. That keeps the rotational motion in your hips — that’s good — and away from your low back — that would be bad.