You see the dumbbell side bend all the time in the gym, but this exercise doesn't make biomechanical sense. There's not much resistance when you're standing upright holding a dumbbell because the dumbbell is very close to your body, giving you a huge mechanical advantage over the weight. And many people even do it by holding two dumbbells, one on each side:
But the weight on one side offsets the weight on the other, making this exercise pretty ineffective at loading the lateral flexors of the torso.
The Right Way
Grab the handle of a cable (or resistance band) that has a low attachment, roughly ankle level with your right hand. Stand tall with your cable at your right side with your feet about shoulder-width apart. Make sure you're standing far enough away from the cable or band so that it's at roughly a 45-degree angle to the floor.
Without rotating your body, slightly bend your torso sideways to the right until you feel a mild stretch in the left side of your torso. Reverse the action and finish the rep by slightly flexing your torso to the left against the resistance.
When doing this exercise with a cable or stretch band, the angle of the cable forces you to work hard to stay upright and maintain that position between reps, giving you more time under tension through the range of motion.
Now, you could certainly hold very heavy dumbbells, but they may exceed your grip strength. You can create the same training effect with a cable and use much lower loads.