This exercise requires the use of the latissimus dorsi muscle (lats) and secondarily affects the trapezoid and rhomboid muscles, the deep back muscles, rear part of the deltoid muscle, the biceps and the forearms.
Stand in a straddle in shoulder width, bend your torso, abs should be firm. Slightly bend your knees, grab the weight bar 15-20 cm wider than shoulder width on both sides so that your palms face your tibia. Arms should be straight, then lead the motion with your elbow and lift the weight bar until the point where it reaches the upper part of your stomach and the lower part of your chest. Let the bar go back to the starting position then repeat the exercise.
In case of most exercises performed with two dumbbells but especially in case of rowing with bent torso you have to change the grip width from time to time. This will provide different difficulty for the exercises and you will develop better. Also you should pay attention that when working out you should focus on the feel in the back muscles.