Also called the horizontal plane. Internal and external rotation of the shoulder and hip live here, as do cable chops, Russian twists, med-ball throws, and the rotation of a golf or tennis swing.
Most transverse work in the gym is anti-rotation rather than rotation: a Pallof press asks the torso to refuse to turn against a cable, which is what the trunk actually does when a client carries something heavy on one side.
Program it in seconds, not reps. Three sets of 20 seconds per side on a Pallof press, with a load light enough that the arms stay locked out, does more for trunk control than 30 twists with a plate.